refactor(api): move secret logic to separate package

[ecrupper]

Based off of #615 #754 #809 #828 #829 #847 #848 #849 #851 #853 and #855

This change continues the refactor efforts initially introduced in the above PRs.

This adds a new secret package to the github.com/go-vela/server/api package.

This contains all the same handlers that existed previously but with each of them within their own file.

Moved the unique helper function to the github.com/go-vela/server/util package, as it seems well-placed there, but I'm willing to move it back into the secret package if requested.

Also changed GetSecret to ListSecrets to stay consistent.

[codecov]

Codecov Report

Merging #856 (d0c14df) into main (4ffd2f1) will increase coverage by 1.94%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #856      +/-   ##
==========================================
+ Coverage   64.84%   66.78%   +1.94%     
==========================================
  Files         284      283       -1     
  Lines       14543    14120     -423     
==========================================
  Hits         9430     9430              
+ Misses       4667     4244     -423     
  Partials      446      446