feat: merge next branch into main

[cjquines]

merges build-sdk-action into main, with some changes

changes compared to build-sdk-action:

• versioning starts from 1.0.0, so the first release will be 1.0.0
• the versioning policy is different
• examples are against v1 (which does not exist yet, but will after release please runs and we do the release motions)
  • the actual release flow has been tested on build-sdk-action
• references to the action name have been changed to upload-openapi-spec-action
• the main action has been moved to /build

for compatibility, note that:

• the main action.yml is essentially unchanged
• the main .gitlab-ci.yml is essentially unchanged
• the src/index.ts, which is used to build dist/index.js, is unchanged. the actual dist file is different because the way we build it is different (using esbuild instead of ncc)

[meorphis]

skimmed and LGTM. a little weird to have a top-level action.yml as well as build/action.yml but it's probably the best call for backwards compatibility