Add metadata get/patch subcommands to restatectl

[pcholakov]

This PR introduces a new metadata subcommand with options to retrieve and update values in both "direct" mode working on a local-metadata-store or remotely against both the Restate-native metadata service and Etcd.

The FlexBuffers-encoded values are rendered to the user as JSON. This allows us to use JSON Patch to succinctly express modifications.

Here is how it looks in action:

> restatectl metadata get -k "schema_registry"
{
  "version": 9,
  "deployments": [
    [
      "dp_11Oa6uG2mNPjTYcD03AFgnn",
      {
        "metadata": {
          "created_at": 1727447806006,
          "delivery_options": {
            "additional_headers": {}
          },
          "supported_protocol_versions": {
            "end": 2,
            "start": 1
          },
          ...

> restatectl metadata patch --access-mode direct --config-file config.toml --key "schema_registry" \
  --patch '[{"op": "replace", "path": "/deployments/0/1/metadata/supported_protocol_versions/end", "value": 1}]' --version 7 --dry-run
Error: Version mismatch: expected v7, got v9 from store

> restatectl metadata patch -c config.toml -k "schema_registry" -
  -p '[{"op": "replace", "path": "/deployments/0/1/metadata/supported_protocol_versions/end", "value": 1}]' -e 9
{
  "version": 10,
  "deployments": [
    [
      "dp_11Oa6uG2mNPjTYcD03AFgnn",
      {
        "metadata": {
          "created_at": 1727447806006,
          "delivery_options": {
            "additional_headers": {}
          },
          "supported_protocol_versions": {
            "end": 1,
            "start": 1
          },
          ...