introducing towncrier to generate changelog from fragments

[emdneto]

Fix #4307
Hopefully, also Fix #4309

Description

Towncrier is a tool adopted in some well-known python projects to generate release notes from fragments. In otel-python we are always dealing with CHANGELOG.mdconflicts during rebases, specially after releases. So, this can help us with it since now we have a changelog entry for each PR in the .changelog directory. This is similar to chloggen adopted in opentelemetry-collector.

How it works:

1. Creating a Changelog Fragment:

   • Contributors add a changelog entry under the .changelog directory in the format <PR_NUMBER>.<CATEGORY>, where <CATEGORY> can be added, changed, fixed, etc.
   • For example, a PR #1234 fixing a bug would create a file named ./.changelog/1234.fixed containing a short description of the change.
   • Every PR should include at least one changelog fragment unless explicitly exempted (e.g., PR's with Skip Changelog).

2. Releasing:

   • During the release, we run towncrier build --yes. This command:
     • Gathers all fragments in the .changelog directory.
     • Renders the release notes.
     • Updates CHANGELOG.md with the generated content.

The configuration is pretty simple and we can customize the changelog as we want using jinja templates and also customize the category names.

I tested the changes using https://github.com/nektos/act: act -W '.github/workflows/changelog.yml' --eventpath event.json

{
    "act": true,
    "env": {
        "GITHUB_REF_NAME": "main"
    },
    "pull_request": {
      "number": 4371,
      "head": {
        "ref": "xyz",
        "sha": "1234567890abcdef1234567890abcdef12345678"
      },
      "base": {
        "ref": "main"
      },
      "user": {
        "login": "opentelemetrybot"
      }
    }
  }

If you checkout this branch and run towncrier build --yes --version=1.30.0/0.51b0
The generated changelog for this branch will be:

## Version 1.30.0/0.51b0 (2025-01-09)

### Changed

- sdk: don't log or print warnings when the SDK has been disabled
  ([#4371](https://github.com/open-telemetry/opentelemetry-python/pull/4371))

### Added

- Add Python 3.13 support
  ([#4353](https://github.com/open-telemetry/opentelemetry-python/pull/4353))
- Add `attributes` field in `metrics.get_meter` wrapper function
  ([#4364](https://github.com/open-telemetry/opentelemetry-python/pull/4364))

[emdneto]

We can probably leave this for another PR to not introduce too many changes at this moment