feat: add requirements.extras for optional dependency groups (CEP 44)

[ytausch]

Summary

Adds the extras key under requirements, as specified by CEP 44 — Optional dependency groups (accepted via conda/ceps#165).

CEP 44 requires recipe formats v1+ to expose requirements.extras (sibling to build, host, run, …), rendering to dict[str, list[MatchSpec]]. The published JSON Schema was lagging: Requirements sets additionalProperties: false but did not declare extras, so editors incorrectly flagged it.

Example

requirements:
  run:
    - main-dependency
  extras:
    diff:
      - textual-diff-view >=0.1.5,<0.2

Selecting the group downstream uses the V3 MatchSpec syntax, e.g. pixi-browse[extras=[diff]].

[Hofer-Julian]

Thank you!

[ytausch]

@Hofer-Julian anything preventing a merge here?