feat: add PR link support to changelog schema

[agrawroh]

Description

This PR add some schema changes to support PR links in Envoy change-logs.

Example Usage

In a changelog entry, you can now include a PR(s) field called prs:

behavior_changes:
- area: aws_iam
  change: |
    As announced in November 2024, the grpc_credentials/aws_iam extension is being deleted.
    Any configuration referencing this extension will fail to load.
  prs: ["54321", "67890"]

Or,

behavior_changes:
- area: aws_iam
  change: |
    As announced in November 2024, the grpc_credentials/aws_iam extension is being deleted.
    Any configuration referencing this extension will fail to load.
  prs:
  - 12345
  - 67890

As next steps, to automate the process of adding PR links to change-log entries, we can create a GitHub workflow that adds PR links to change-log entries when PRs are merged.

[phlax]

thanks @agrawroh

i think this should probably be prs - ie associated to prs rather than locked to a single one

[phlax]

sorry for delay in reviewing/testing this

i think this new field should be a list of ints as that is kinda what is expected

currently its not used anywhere - but im guesing that when hooked up the docs will display the list of linked prs in the docs - happy for that to be addressed separately

ftr, i tested this with:

$ bazel run @envoy_repo//:sync
$ bazel run @envoy_repo//:release
$ bazel run @envoy_repo//:dev

... from envoy repo - all work correctly assuming the schema is correct, and i also built the docs from changelog with new schema