bug: low level api operations don't work without calling describe_table first

[bpsoos]

The issue

Based on the docs it seems that we should be able to instantiate a Connection or a TableConnection and perform put_item, get_item, update_item and delete_item operations on them. However these all fail with the following error:
Meta-table for '<table name>' not initialized
These operations work if we call describe_table first on the connection, like in the current integration tests.

examples to reproduce the issue:

    table_name = 'pynamodb-ci-connection'
    conn = Connection(host=ddb_url)
    # conn.describe_table(table_name)  # bug: operations don't work without calling describe_table first

    conn.put_item(
        table_name,
        'item1-hash',
        attributes={'foo': {'S': 'bar'}},
    )  # pynamodb.exceptions.TableError: Meta-table for 'pynamodb-ci-connection' not initialized

   table_name = 'pynamodb-ci-connection'
   conn = TableConnection(table_name=table, host=ddb_url)
   # conn.describe_table()  # bug: operations don't work without calling describe_table first
   conn.put_item(
       'item1-hash',
       attributes={'foo': {'S': 'bar'}},
   )  # pynamodb.exceptions.TableError: Meta-table for 'pynamodb-ci-connection' not initialized

See the integration tests in the linked pull request for runnable versions of these examples.

Root cause

Itt seems that Connection.get_identifier_map relies on a MetaTable corresponding to the given table to extract information about it's hash and range keys.

Solution proposal

I propose to add an optional parameter, keyname_details, to all of the relevant operations and to Connection.get_identifier_map. If keyname_details is given use that, otherwise default to the current behaviour, that is try to fetch the key details from a MetaTable.

This could look something like the following:

@dataclass
class TypedName:
    name: str
    name_type: str


@dataclass
class KeyNameDetails:
    hash_keyname: TypedName
    range_keyname: Optional[TypedName] = None


class Connection(object):
    ...
    def get_item(
        self,
        table_name: str,
        hash_key: str,
        range_key: Optional[str] = None,
        consistent_read: bool = False,
        attributes_to_get: Optional[Any] = None,
        keyname_details: Optional[KeyNameDetails] = None,
    ) -> Dict:
        operation_kwargs = self.get_operation_kwargs(
            table_name=table_name,
            hash_key=hash_key,
            range_key=range_key,
            consistent_read=consistent_read,
            attributes_to_get=attributes_to_get
            keyname_details=keyname_details,
        )
        ...

    def get_operation_kwargs(
        self,
        table_name: str,
        hash_key: str,
        range_key: Optional[str] = None,
        key: str = KEY,
        attributes: Optional[Any] = None,
        attributes_to_get: Optional[Any] = None,
        actions: Optional[Sequence[Action]] = None,
        condition: Optional[Condition] = None,
        consistent_read: Optional[bool] = None,
        keyname_details: Optional[KeyNameDetails] = None,
        return_values: Optional[str] = None,
        return_consumed_capacity: Optional[str] = None,
        return_item_collection_metrics: Optional[str] = None,
        return_values_on_condition_failure: Optional[str] = None
    ) -> Dict:
        ...
        operation_kwargs.update(
            self.get_identifier_map(
                table_name,
                hash_key,
                range_key,
                key=key,
                keyname_details= keyname_details,
            ),
        )

Connection.get_identifier_map:

 class Connection(object):
    ...
        def get_identifier_map(
        self,
        table_name: str,
        hash_key: str,
        range_key: Optional[str] = None,
        key: str = KEY,
        keyname_details: Optional[KeyNameDetails] = None,
    ) -> Dict:
        if keyname_details is None:
            tbl = self.get_meta_table(table_name)
            if tbl is None:
                raise TableError("No such table {}".format(table_name))
            return tbl.get_identifier_map(hash_key, range_key=range_key, key=key)

        kwargs: Dict[str, Any] = {
            key: {
                keyname_details.hash_keyname.name: {
                    keyname_details.hash_keyname.name_type: hash_key
                }
            }
        }
        if keyname_details.range_keyname is not None:
            kwargs[key][keyname_details.range_keyname.name] = {
                keyname_details.range_keyname.name_type: range_key
            }
        return kwargs