feat: add buf plugin to generate openapiv2 spec

[lincolnthalles]

I was unable to use the plugin opt openapi_configuration to supply the additional API configuration file (it simply doesn't find the file), but it's a matter of API users replacing the first 4 lines of v2.swagger.yaml with something like this:

swagger: "2.0"
host: "demo.usememos.com"
schemes:
  - https
  - http
securityDefinitions:
  Bearer:
    type: apiKey
    in: header
    name: Authorization
    description: >-
      Enter the Memos token with the `Bearer` prefix, e.g. "Bearer abcde12345"
security:
  - Bearer: []
info:
  title: Memos API
  version: 2.0.0

This enables interacting with v2 API using Insomnia built-in Swagger-UI or even using the web-hosted Swagger Editor.

By the way, there's not much control of file name, the plugin will always append ".swagger.yaml".

It's just a matter of running buf generate after changes on v2 API and commiting v2.swagger.yaml to keep the spec updated.

[boojack]

LGTM