feat: add build.post_process to recipe-format schema

[mohitdebian]

Adds build.post_process to the canonical recipe-format schema.

This aligns the recipe specification with functionality already implemented
in rattler-build, where build.post_process allows regex-based text
replacements on files during packaging.

The schema introduces a PostProcess object with required fields:

• files
• regex
• replacement

Includes a valid example recipe and schema validation tests.

Follow-up to the documentation merged in rattler-build.

[mohitdebian]

This follows up on the documentation added in prefix-dev/rattler-build#2254

[mohitdebian]

@wolfv this is the follow up PR adding build.post_process to the recipe-format schema as suggested in prefix-dev/rattler-build#2254. CI is waiting for workflow approval