Add method to return edge indices from endpoints

[eendebakpt]

Fixes #978

[mtreinish]

Overall this LGTM, it does overlap with #1011 but that PR has been idle with open review comments for a couple months now and this addresses the feedback on that PR. I think the only thing still to do on this PR is fix the CI failure (it just needs to have black run as there is excess whitespace in the digraph tests) and also this could use a release note to document the new methods being added in this PR. The process for adding a release note is documented here: https://github.com/Qiskit/rustworkx/blob/main/CONTRIBUTING.md#release-notes

[coveralls]

Pull Request Test Coverage Report for Build 7589534946

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 95.966%

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Totals
Change from base Build 7578823264:	0.04%
Covered Lines:	16056
Relevant Lines:	16731

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💛 - Coveralls