feat: auth expire self / logout

[markphelps]

Fixes: FLI-172

• Adds new grpc method auth.ExpireAuthenticationSelf that takes optional expires_at param to update the current authentication expiration date, defaults to now if not set
• Adds HTTP middleware wrapper to support clearing the flipt_client_token and flipt_client_state cookies to support logout functionality in the UI

I made it a PUT because that seemed the most appropriate, even though it isn't technically idempotent since continuos calling (without the optional expiresAt field) will result in expiresAt getting set to NOW() each time). Open to suggestion though

[GeorgeMac]

On the PUT vs GET this is definitely something to consider. PUT could be better in some ways.

I couldn't really make the callback a PUT or POST because the callback is a target that an external provider redirects the browser to.
Using GET means you can navigate a browser to the path and get the behaviour.
Is this something we desire for Flipt? As in, the ability to navigate to this expire route and logout.

It is not really an abusable thing given out setup. The flipt_client_token cookie is set with SameSite: strict.
So, an attacker couldn't force someone to logout. Because they would navigate from another domain and the browser would omit the cookie.
TL;DR GET wouldn't be abusable. But it is just the experience we want.

[GeorgeMac]

Nice. I think this looks great.

My feedback is mostly at our discretion to take it or leave it. So I have approved.

[codecov-commenter]

Codecov Report

Merging #1279 (88b6a24) into main (4902107) will decrease coverage by 0.12%.
The diff coverage is 82.75%.

@@            Coverage Diff             @@
##             main    #1279      +/-   ##
==========================================
- Coverage   79.82%   79.70%   -0.12%     
==========================================
  Files          42       43       +1     
  Lines        3217     3267      +50     
==========================================
+ Hits         2568     2604      +36     
- Misses        521      531      +10     
- Partials      128      132       +4     

Impacted Files	Coverage Δ
internal/storage/auth/auth.go	19.23% <ø> (ø)
internal/storage/auth/memory/store.go	77.09% <66.66%> (-0.77%)	⬇️
internal/server/auth/method/oidc/http.go	83.50% <75.00%> (ø)
internal/server/auth/http.go	81.81% <81.81%> (ø)
internal/server/auth/server.go	82.00% <88.88%> (+1.51%)	⬆️
internal/storage/auth/sql/store.go	88.60% <100.00%> (+0.50%)	⬆️
internal/storage/oplock/sql/sql.go	90.82% <0.00%> (-5.51%)	⬇️

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