feat(oauth): add possibility to specify OAuthLogoutEndpoint for logout from OAuth Identity provider

[bednar]

Related to https://github.com/influxdata/EAR/issues/4786

What was the problem?

The user remains logged in to the OAuth identity provider even after logging out from Chronograf. Therefore, during the next login to Chronograf, the user isn't prompted to enter credentials again.

What was the solution?

Chronograf now includes a new configuration property, OAuthLogoutEndpoint, which allows you to specify a logout callback. This callback is used to log out users from both Chronograf and their OAuth Identity Provider.

Customers will need to configure this property based on their respective identity providers. Below are the documentation links for a few common providers:

• AWS Cognito: Logout Endpoint
• IBM Cloud App ID: Logout Cloud Directory SSO
• Auth0: Auth0 Logout
• Microsoft ADFS: Single log-out for OpenID Connect with AD FS

For instance, for a localhost instance of Chronograf using Auth0 as the provider, the configuration would look like this:

export OAUTH_LOGOUT_ENDPOINT=https://dev-xyz.us.auth0.com/v2/logout?returnTo=http://localhost:8080&client_id=abc

• [x] CHANGELOG.md updated with a link to the PR (not the Issue)
• [x] Well-formatted commit messages
• [x] Rebased/mergeable
• [x] Tests pass
• [x] Sign CLA (if not already signed)