feat: get memberships of an organisation by username

[fv316]

Description

This PR creates a new GHOrganisation method that allows for fetching the GHMembership object from a String username. This also means that GHUser objects (inside the GHMembership object) can be fetched directly from a username instead of using the existing listMembers method.

Fixes #1849

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.66%. Comparing base (c5ce1f6) to head (24105f7).
Report is 7 commits behind head on main.

❗ Current head 24105f7 differs from pull request most recent head b74126c

Please upload reports for the commit b74126c to get more accurate results.

Additional details and impacted files

@@ Coverage Diff @@ ## main #1850 +/- ## ============================================ + Coverage 80.64% 80.66% +0.02%  - Complexity 2358 2360 +2  ============================================ Files 225 225 Lines 7203 7206 +3 Branches 395 395 ============================================ + Hits 5809 5813 +4  + Misses 1149 1148 -1  Partials 245 245 

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