feat(leiden): expose additional parameter

1) expose 'random_state' parameter to better align with Scanpy
2) pass it to cuGraph via 'culeiden'
3) save its value in '.uns["leiden"]["params"]'
4) describe it in docstring
5) make multiple small improvements to docstring

N.b., the 'theta' parameter was not exposed because a bug in cuGraph (present at commit 1e446c4) makes it have no effect; this should be re-visited when cuGraph is updated.

for more information, see https://pre-commit.ci

fix(leiden): fix undefined type

Import 'Union' from 'typing' to fix error when defining 'random_state' (line 17)

Release note describing addition of `random_state` parameter to `leiden`

Include 0.9.4.md