Allow billable member sorting by last activity

What does this MR do?

As discussed in #330055 (closed), we are going to allow sorting of billable members within the Seat Usage view by the member last_activity_on attribute.

This MR is the backend changes neccessary to support that effort by adding the attribute to the sorting helper for the REST API.

Testing

curl --request GET \
  --url 'http://127.0.0.1:3000/api/v4/groups/:id/billable_members?sort=last_activity_on_asc' \
  --header 'PRIVATE-TOKEN: <access token>'

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.