Add optional support for serde with a feature flag

[firstdorsal]

I would like to parse a JSON/YAML document with serde into the required enums for the management API.

For example:
OidcAppType as one of Web, UserAgent or Native

{
    "appType": "Web"
}

This maybe could be done like mentioned here: https://github.com/neoeinstein/protoc-gen-prost/blob/main/protoc-gen-prost-serde/README.md

cargo install protoc-gen-prost-serde

# buf.gen.yaml
version: v1
managed:
  enabled: true
plugins:
  - plugin: prost
    out: src/api/generated
    opt:
      - compile_well_known_types
      - extern_path=.google.protobuf=::pbjson_types
  - plugin: prost-serde
    out: src/api/generated
  - plugin: tonic
    out: src/api/generated
    opt:
      - compile_well_known_types
      - extern_path=.google.protobuf=::pbjson_types
      - no_server
  - name: prost-crate
    out: .
    strategy: all
    opt:
      - gen_crate
      - include_file=src/api/generated/mod.rs

and adding pbjson = { version = "0.7.0", optional = true } as dependency

Support for schemars::JsonSchema would also be great although I didn't find a simple way to do this like for serde.