fix(cleanup): ensure all methods register their cleanup schedules

[GeorgeMac]

It was observed that OIDC method-created authentications were not getting cleared away.
The cleanup process when created was hardcoded to only perform cleanup for METHOD_TOKEN (mistakenly).

This was an easy mistake (not registering the method in all the places). It was why I added the generic method container and the AllMethods() support to ensure that adding new methods was as simple as ensuring the methods returns it info here:

flipt/internal/config/authentication.go

Lines 168 to 171 in 6144f32

	return []StaticAuthenticationMethodInfo{
	a.Token.Info(),
	a.OIDC.Info(),
	}

The types should depend on performing background operations of this AllMethods() list. Instead of having to manually add new methods everywhere they're created.

However, I never updated cleanup to take advantage of this.
This change does that. It updates the cleanup process to walk this list and create a cleanup goroutine per method.

Additionally, the cleanup test ensures that each method defined on all methods is configure and tested.

[codecov-commenter]

Codecov Report

Merging #1337 (3cda11e) into main (6144f32) will decrease coverage by 0.35%.
The diff coverage is 37.50%.

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@@            Coverage Diff             @@
##             main    #1337      +/-   ##
==========================================
- Coverage   80.81%   80.47%   -0.35%     
==========================================
  Files          43       43              
  Lines        3336     3344       +8     
==========================================
- Hits         2696     2691       -5     
- Misses        512      523      +11     
- Partials      128      130       +2     

Impacted Files	Coverage Δ
internal/config/authentication.go	76.66% <23.07%> (-6.97%)	⬇️
internal/cleanup/cleanup.go	71.66% <54.54%> (-5.76%)	⬇️

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[markphelps]

couple minor suggestions. but otherwise lgtm.

I think we should prob create a release/1.18 branch and rebase this onto the v1.18.1 tag in order to get this out in v1.18.2 today.

[GeorgeMac]

Agreed @markphelps 👍 (on all counts)

I adjusted the PR based on your feedback.