feat: adds flex groups dropdown menu in assignment modal

[katrinan029]

Description

Adds support for admins to select flex groups from the dropdown menu to the Learner credit assignment modal. The members associated with that group will be added to the summary tab once the user clicks the Apply selections button.
If a group does not have any flex groups, the dropdown will not be shown.

https://2u-internal.atlassian.net/browse/ENT-9491

Screen.Recording.2024-10-22.at.4.52.11.PM.mov

Test scenarios

1. checkout this branch and visit customer page https://localhost.stage.edx.org:1991/alc-general/admin/learner-credit/7f344aea-c28c-4ef0-95e6-fb6e58f98495/assign to choose one of the courses to assign learners to.
2. verify that if the user clicks on a group but does not the Apply selections button and navigates away from the dropdown menu, the selections will clear. The same should apply when the user unchecks a group that was previously added to the summary tab -- previous selections/unselections should persist until the user clicks the 'Apply selections' button.

For all changes

• Ensure adequate tests are in place (or reviewed existing tests cover changes)

Only if submitting a visual change

• Ensure to attach screenshots
• Ensure to have UX team confirm screenshots

[codecov]

Codecov Report

Attention: Patch coverage is 82.41758% with 16 lines in your changes missing coverage. Please review.

Project coverage is 85.70%. Comparing base (4f9d197) to head (8c81642).

Files with missing lines	Patch %	Lines
...it-management/data/hooks/useGroupDropdownToggle.js	75.60%	6 Missing and 4 partials ⚠️
...t-management/data/hooks/useEnterpriseFlexGroups.js	45.45%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1335      +/-   ##
==========================================
- Coverage   85.72%   85.70%   -0.03%     
==========================================
  Files         594      597       +3     
  Lines       12977    13068      +91     
  Branches     2704     2759      +55     
==========================================
+ Hits        11125    11200      +75     
- Misses       1792     1802      +10     
- Partials       60       66       +6     

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