refactor(api): move auth logic to separate package

[cognifloyd]

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

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

This adds a new auth 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.

There are several top-level auth-related handlers, so I combined them into the same package, and I renamed some of the handlers.

Here is a summary of handler moves and renames:

Endpoint	Old File	Old Handler Name	New File	New Handler Name
GET /authenticate	api/authenticate.go	Authenticate	api/auth/get_token.go	GetAuthToken
GET /authenticate/:type[/:port]	api/authenticate.go	AuthenticateType	api/auth/redirect.go	GetAuthRedirect
POST /authenticate	api/authenticate.go	AuthenticateToken	api/auth/post_token.go	PostAuthToken
GET /login	api/login.go	Login	api/auth/login.go	Login
GET /logout	api/logout.go	Logout	api/auth/logout.go	Logout
GET /token-refresh	api/token.go	RefreshAccessToken	api/auth/refresh.go	RefreshAccessToken
GET /validate-token	api/token.go	ValidateServerToken	api/auth/validate.go	ValidateServerToken

NB: The commits are arranged to preserve git history (git blame) for all files. I suspect that the squash commit will erase that, but oh well - the history is at least preserved in this branch.

[codecov]

Codecov Report

Merging #853 (eb1c038) into main (a374e2b) will increase coverage by 1.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #853      +/-   ##
==========================================
+ Coverage   62.03%   63.10%   +1.07%     
==========================================
  Files         289      285       -4     
  Lines       15202    14944     -258     
==========================================
  Hits         9430     9430              
+ Misses       5326     5068     -258     
  Partials      446      446              

see 3 files with indirect coverage changes

[ecrupper]

Awesome — I like it. One thing I did notice that was from an old commit that would be nice to change with this refactor