Fix API for numeric paths and usernames

What does this MR do?

Groups and Projects can have numeric names, and Users can have numeric usernames, meaning they have numeric paths. Previously the API assumed a numeric parameter was an ID, and would fail to return data for anything with a numeric path. This fixes that bug by falling back to a path lookup if an ID lookup fails.

Performance note: In the case of numeric names, an additional database request will be performed. Because numeric names are uncommon, and the additional database request is not remarkable, this is expected to have negligible performance impact.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #331896 (closed)