Include PR/Issue links in the changelog

[joaopgrassi]

Fixes #1627

Changes

Adds a template for customizing the generated changelog file by chloggen. The template is a copy of the default, but renders links for issues/PR numbers specified in the issues field in the changelog entries.

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

[joaopgrassi]

Here's how it will look on current main

Generated changelog updates for user:
## vTODO

### 🛑 Breaking changes 🛑

- `vcs`: Add the VCS metrics inspired by the GitHub Receiver ([#1372](https://github.com/open-telemetry/semantic-conventions/issues/1372))
  Makes the following changes:

  - Add metrics `vcs.change.count`, `vcs.change.duration`, `vcs.change.time_to_approval`, `vcs.repository.count`, `vcs.ref.count`,
    `vcs.ref.lines_delta`, `vcs.ref.revisions_delta`, `vcs.ref.time`, `vcs.contributor.count`
  - The VCS attributes `vcs.change.state`, `vcs.revision_delta.direction` and `vcs.line_change.type` have been added to the registry.
  - The VCS ref attributes have been duplicated to `vcs.ref.base.*` to allow for ref comparisons.
  - The VCS attribute `vcs.ref.type` has been added for simplicity when neither a full head or base ref is necessary.
  - `vcs.repository.change.*` attributes have been deprecated and moved to `vcs.change.*`.
  - `vcs.repository.ref.*` attributes have been deprecated and moved to `vcs.ref.head.*`.

Note: The links will only be added from now on.

[lmolkova]

Thank you!

[lmolkova]

@joaopgrassi I'd merge, but I can't update your branch :(