Add proto metadata options for chunking

[lovromazgon]

Description

A follow-up to #204 which adds options for the introduced metadata fields, so that the metadata field names are part of the proto definition. Also updates the test that ensures Go constants match the proto constants.

Note that buf breaking will complain, because the file option types changed. Those are not meant to represent the type (all types in the metadata are strings anyway), we're just using those options to make sure the metadata constants are defined in the proto file and are accessible through code. Kinda hacky, but hey, there's no other way to do constants in proto files.

Quick checks:

• I have followed the Code Guidelines.
• There is no other pull request for the same update/change.
• I have written unit tests.
• I have made sure that the PR is of reasonable size and can be easily reviewed.

[github-actions]

The latest Buf updates on your PR. Results from workflow buf-validate / validate (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	❌ failed (4)	May 16, 2025, 11:49 AM