Allow users to override TUF repository URL prefix

[trishankatdatadog]

What does this PR do?

Allow users to override TUF repository URL prefix.

Motivation

So that you can override where the repository resides, if needed (e.g., for localhost testing).

Additional Notes

N/A.

Review checklist (to be filled by reviewers)

• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached
• Feature or bugfix must have tests
• Git history must be clean
• If PR adds a configuration option, it must be added to the configuration file.

[codecov]

Codecov Report

Merging #3660 into master will decrease coverage by 5.69%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3660      +/-   ##
==========================================
- Coverage   86.52%   80.83%    -5.7%     
==========================================
  Files         728        8     -720     
  Lines       37654      240   -37414     
  Branches     4495       22    -4473     
==========================================
- Hits        32582      194   -32388     
+ Misses       3850       36    -3814     
+ Partials     1222       10    -1212