[FEATURE] Make Template Writeable

[dbwiddis]

Is your feature request related to a problem?

The Template class does not properly impement Writeable:

• there is no constructor taking StreamInput
• the writeTo() implementation does not handle the List<Version>

// TODO: fix writeable when implementing get workflow API
@Override
public void writeTo(StreamOutput output) throws IOException {
    output.writeString(name);
    output.writeOptionalString(description);
    output.writeString(useCase);
    output.writeVersion(templateVersion);
    // output.writeList((List<? extends Writeable>) compatibilityVersion);
    output.writeMapWithConsistentOrder(workflows);
    if (user != null) {
        output.writeBoolean(true); // user exists
        user.writeTo(output);
    } else {
        output.writeBoolean(false); // user does not exist
    }
}

This is a similar issue to the resource mapping in #358 where output.writeList() needs a type that is supported in writeGenericValue. See StreamOutput line 663. These do not include the Version class.

The Version class itself (if not in a collection) can be written (it just writes the integer ID).

What solution would you like?

Option 1: Just don't implement writeable. We currently pass the template in the Response as a JSON string, parsing it at the other end.

Option 2: Properly make the whole template writeable so we don't have to use the string workaround. This should have improved performance.

For the specific TODO:

Manually write the list:

output.writeVInt(compatibilityVersion.size());
for (Version v : compatibilityVersion) {
  output.writeVersion(v);
}

And implement the corresponding readers in the constructor.

[joshpalis]

Going with option 1 since we don't ever write the template object itself to a stream, only the template json