[CI] Improve Release Automation and Changelog about dynaconf HOT 1 CLOSED

Implement get-gh-username on git-changelog, so if user set some config like get-gh-username=True the tool calls GH api and stores this info in the Commit object property.

Then, in the template I can use the actual GitHub user reference and links, which looks better in the github Release Notes:

# in git-changelog jinja template:
@{{ commit.gh_username }} -- {{ commit.gh_username_url }}

# renders:
@pedro-psb -- https://github.com/pedro-psb"

To get the github username of a commit author it is possible to call the GitHub API as so:

gh api \
  -H "Accept: application/vnd.github+json" \
  -H "X-GitHub-Api-Version: 2022-11-28" \
  /repos/dynaconf/dynaconf/commits/REF

from dynaconf.