sessions pkg: minor refactors

[Jusshersmith]

Problem

In an effort to reduce the scale of some other changes I'm trying to pull through, we make some small refactors here to internal/proxy/providers/sso.go.

Solution

Included refactors are:

• Pull out the withinGracePeriod method within internal/proxy/providers/sso.go into a new method within the sessions package: IsWithinGracePeriod.
• Instead of calling a local extendDeadline function within internal/proxy/providers/sso.go, call the already existing sessions package ExtendDeadline method.

[codecov]

Codecov Report

Merging #322 (d2371e9) into main (a1b1b74) will increase coverage by 0.20%.
The diff coverage is 64.70%.

@@            Coverage Diff             @@
##             main     #322      +/-   ##
==========================================
+ Coverage   62.73%   62.94%   +0.20%     
==========================================
  Files          58       58              
  Lines        4286     4758     +472     
==========================================
+ Hits         2689     2995     +306     
- Misses       1382     1546     +164     
- Partials      215      217       +2     

Impacted Files	Coverage Δ
internal/pkg/sessions/session_state.go	79.16% <50.00%> (-5.45%)	⬇️
internal/proxy/providers/sso.go	68.89% <69.23%> (+0.99%)	⬆️
internal/auth/providers/group_cache.go	57.14% <0.00%> (-11.28%)	⬇️
internal/auth/logging_handler.go	28.12% <0.00%> (-2.07%)	⬇️
internal/proxy/logging_handler.go	14.75% <0.00%> (-1.58%)	⬇️
internal/pkg/httpserver/httpserver.go	66.66% <0.00%> (-1.20%)	⬇️
internal/auth/providers/google.go	58.49% <0.00%> (-0.66%)	⬇️
internal/auth/error.go	72.22% <0.00%> (-0.51%)	⬇️
internal/auth/mux.go	74.54% <0.00%> (-0.46%)	⬇️
... and 48 more

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[laurenzhang]

these changes lgtm!