Add stubs for pygit2 (#11374)

The upstream library is very tricky to type (likely requires nontrivial
refactoring), and only contains partial type information, but stubs are
a lot easier because only the public signatures are involved this way,
so I plan to first make the library usable in typed projects by making
stubs available here, then gradually work my way upstream.

The stubs are auto-generated then completed with fully manual inspection
of every Python source file. The `_pygit2.pyi` comes from upstream and
is mostly untouched except for required style changes, the signature of
`options()`, and `FilterSource` which is missing from upstream.

stubs/pygit2/@tests/stubtest_allowlist.txt

@@ -0,0 +1,17 @@
+# stubtest wrongly checks the Python implementation with the cffi type's
+# stubs, likely due to name shadowing that happens at import-time
+pygit2.Repository.__init__
+pygit2.Repository._from_c
+# @cached_property that fail to get recognized as such by stubtest
+pygit2.config.ConfigEntry.level
+pygit2.config.ConfigEntry.raw_name
+pygit2.config.ConfigEntry.raw_value
+# Actual signatures enforced by C-level checks not visible to stubtest
+pygit2.Mailmap.__init__
+pygit2.OdbBackend.__init__
+pygit2.RefLogEntry.__init__
+pygit2.RefdbBackend.__init__
+pygit2._pygit2.Mailmap.__init__
+pygit2._pygit2.OdbBackend.__init__
+pygit2._pygit2.RefLogEntry.__init__
+pygit2._pygit2.RefdbBackend.__init__