Add support to export flat json schemas

[Dogacel]

Motivation:

Nested JSON schemas might be hard to read. Especially if we want to introduce OpenAPI spec generation, the ability to generate the $defs block independently can be super useful.

Modifications:

• Add flat option to the json schema generation for method information.
• Add a method to generate $defs block for a service specification based on all structs.
• Slight refactoring to extract visited out-of-function signatures to the class body.
• Omit description if empty. (Gist is not up-to-date, but should be fine because unit tests verify).

Result:

• Closes #4743
• See generated schemas for grpc service here: https://gist.github.com/Dogacel/bfa94713eb3202d4a1f02f057ad1a066

[Dogacel]

I know it has been a year however I had a second look into this PR and it LGTM 🙂

I would like to work on fixing the JSON suggestions for Well known types as described in the official documentation: https://protobuf.dev/programming-guides/proto3/#json

Please LMK if it is OK to merge this and for me to continue working on fixing the JSON schema problems.

@minwoox

[github-actions[bot]]

🔍 Build Scan® (commit: 9319c0cbf673e53e6835f950ccaee678457d996a)

Job name	Status	Build Scan®
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build-self-hosted-unsafe-jdk-8	✅	https://ge.armeria.dev/s/u2ji7mf425hxq
build-self-hosted-unsafe-jdk-21-snapshot-blockhound	✅	https://ge.armeria.dev/s/32myz3y5mf2bs
build-self-hosted-unsafe-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/xphzlfqnqqrjo
build-self-hosted-unsafe-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/zwm6lady6mxnq
build-self-hosted-unsafe-jdk-17-leak	✅	https://ge.armeria.dev/s/d5m2kwmnjodiu
build-self-hosted-unsafe-jdk-11	✅	https://ge.armeria.dev/s/z44cwfoiciclm
build-macos-12-jdk-21	✅	https://ge.armeria.dev/s/f7aepgyju7vbo

[Dogacel]

LGTM and sorry for taking so long until I review this PR, @Dogacel 😓

No problem, I kinda left it as it is last year because I wasn't planning to work on DocService related stuff anytime soon 🙂