[MINT-3235] Get milestones and solutions by relation loaders and helper methods

[StevenWadeOddball]

MINT-3235

Description

1. This is a follow up PR to [MINT-3235] Get milestones and solutions by relation loaders #1557
2. Unit tests for getting solutions and milestones
3. Updated mapping function to allow a custom maptype

How to test this change

1. Use postman to create a common milestone with common solutions
2. Use the query to return milestones and their solutions,
   a. ensure that the new common solutions are returned
   b. ensure the nested milestone returned is that milestone

PR Author Checklist

• I have provided a detailed description of the changes in this PR.
• I have provided clear instructions on how to test the changes in this PR.
• I have updated tests or written new tests as appropriate in this PR.
• Updated the Postman Collection if necessary.

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[OddTomBrooks]

I have no feedback on the PR and functional testing was successful