feat(github): add automatic pull request merging support

Introduce automatic pull request merging capability (`auto_merge_pull_requests`)
to class `Repository` in `github_rest_api/github.py` along with support functions,
validating author/reviewer lists, title Conventional-Commit types, head commit
minimum age, and clean mergeable state. Add comprehensive test coverage in
`tests/test_github.py` and bump package version to 0.44.0.

Author: dclong

github_rest_api/__init__.py

@@ -1,11 +1,19 @@
 """GitHub REST APIs."""
 
 from .github import (
+    MergeMethod,
     Organization,
     Repository,
     RepositoryType,
     SecretVisibility,
     User,
 )
 
-__all__ = ["Organization", "Repository", "RepositoryType", "SecretVisibility", "User"]
+__all__ = [
+    "MergeMethod",
+    "Organization",
+    "Repository",
+    "RepositoryType",
+    "SecretVisibility",
+    "User",
+]