sso_auth: fix client authentication for revoke endpoint

[Jusshersmith]

Problem

/oauth2/sign_out does not currently work with the Okta provider:

An error occurred during sign out. Please try again.

is returned, and the request to sign the user out is unsuccessful.

Solution

This PR adds client authentication for the Revoke function (I think this must have been lost during development?), which is used to sign out via the /oauth2/sign_out endpoint.

Notes

Client authentication is provided using the same method as in the ValidateSessionState function:

sso/internal/auth/providers/okta.go

Lines 117 to 123 in 521ff9c

	form := url.Values{}
	form.Add("token", s.AccessToken)
	form.Add("token_type_hint", "access_token")
	form.Add("client_id", p.ClientID)
	form.Add("client_secret", p.ClientSecret)
	err := p.oktaRequest("POST", p.ValidateURL.String(), form, []string{"action:validate"}, nil, &response)

[Jusshersmith]

I believe we want to revoke the refresh token, which also forces the access token to be revoked:

(https://developer.okta.com/docs/guides/revoke-tokens/overview/)

Revoking only the access token
Revoking only the access token will effectively force the use of the refresh token to retrieve a new access token. This could be useful if, for example, you have changed a user's data and you want this information to be reflected in a new access token.

Revoking only the refresh token
If you revoke only the refresh token then the access token will also be revoked. This allows you to, for example, force a user to reauthenticate.

[codecov]

Codecov Report

Merging #217 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #217      +/-   ##
==========================================
+ Coverage   61.21%   61.23%   +0.01%     
==========================================
  Files          50       50              
  Lines        4071     4073       +2     
==========================================
+ Hits         2492     2494       +2     
  Misses       1393     1393              
  Partials      186      186

Impacted Files	Coverage Δ
internal/auth/providers/okta.go	61.88% <100%> (+0.31%)	⬆️