auth: amazon cognito provider

[jphines]

This adds an additional Identity Provider for Amazon Cognito.

There are some TODO's here:

• Sign out behavior for Amazon Cognito is a bit weird -- there is no endpoint to issues sign-outs except globally.
• There is no validation endpoint -- we make the full profile call to validate user state
• Group behavior is a big TODO. It would be nice if we could make this a custom attribute, but something we should look into
• Documentation

[codecov]

Codecov Report

Merging #250 into master will decrease coverage by 1.17%.
The diff coverage is 48.85%.

@@            Coverage Diff             @@
##           master     #250      +/-   ##
==========================================
- Coverage   62.53%   61.35%   -1.18%     
==========================================
  Files          54       57       +3     
  Lines        4201     4637     +436     
==========================================
+ Hits         2627     2845     +218     
- Misses       1385     1580     +195     
- Partials      189      212      +23

Impacted Files	Coverage Δ
internal/auth/providers/google_mock_admin.go	50% <ø> (ø)
internal/auth/configuration.go	46.97% <0%> (-4.56%)	⬇️
internal/auth/providers/google.go	58.49% <100%> (ø)	⬆️
internal/auth/providers/amazon_cognito_admin.go	22.3% <22.3%> (ø)
...ternal/auth/providers/amazon_cognito_mock_admin.go	33.33% <33.33%> (ø)
internal/auth/providers/amazon_cognito.go	64.31% <64.31%> (ø)
internal/auth/options.go	86.13% <88.88%> (+7.56%)	⬆️

[Jusshersmith]

👋 @jphines - pinging you on this as although you wrote the first portion, a fair chunk (and time) has been added since then, so seems worth another general look over, providing you have the bandwidth?

I think the biggest things that have been added/changed is the group membership validation logic (https://github.com/buzzfeed/sso/pull/250/files?file-filters%5B%5D=.go#diff-ea62f132a9d50a2ecbeb99d1dd8c48e1R340),
and the cognito admin file (https://github.com/buzzfeed/sso/pull/250/files?file-filters%5B%5D=.go#diff-c8e9125622d7201b686c3d99338c8e3aR1). Along with some related configuration additions.

[jphines]

A few comment nits -- but the logic looks good.