Add in and out edge indices functions

[ThisuraGallage]

This PR introduces a new method, in_edge_indices, to the rustworkx.PyDiGraph class. The method returns the indices of incoming edges for a specified node.
#1360

• I ran rustfmt locally
• I have added the tests to cover my changes.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.

I'll add the documentation once the function signature is finalized.

[coveralls]

Pull Request Test Coverage Report for Build 12926634311

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Details

• 40 of 40 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.009%) to 95.82%

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Totals
Change from base Build 12817417491:	0.009%
Covered Lines:	18383
Relevant Lines:	19185

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💛 - Coveralls

[IvanIsCoding]

The current implementation looks good to me. I left some minor nitpick comments but they are not blocking.

What is going to be more work is adding even more methods...

• PyGraph should have the same methods even if the implementation is just calling incident_edges for consistency
• PyDiGraph and PyGraph could use a out_edge_indices

So that is 3 more methods which were in the original request but are useful to maintain some coherence accross the API

[IvanIsCoding]

LGTM but I will run this through @mtreinish before adding four similar methods for a second opinion