Validate credentials in provider configure func

[bendrucker]

This PR adds credential validation when the provider's configuration function is called during initialization. It calls /api/v1/users/me and returns an error when it fails, which will prevent further execution of the provider. This seems like the only API route that should be accessible regardless of administrative roles assigned to the calling user.

When using an invalid token:

curl 'https://api.okta.com/api/v1/users/me' -H "Authorization: SSWS foo"

{
  "errorCode": "E0000011",
  "errorSummary": "Invalid token provided",
  "errorLink": "E0000011",
  "errorId": "oaeKfsPfu75Ts6DqXdSZmlRXg",
  "errorCauses": []
}

This provides some assurance that the credentials are valid without causing a single failing request to crash the plugin and halt other okta_* resources, for which the credentials may be valid.

• Fixes data.okta_user: plugin crashes on expected 403 errors #577
• Reverts Added AuthTransport for handling auth errors #571
• Fixes Error handling for improperly formatted api tokens should hard fail rather than repeat. #558, superseding the reverted PR

[bendrucker]

Here's some examples of how other providers handle this similarly:

• aws
• google

[bendrucker]

Thanks for the quick turn around!

[bogdanprodan-okta]

@bendrucker no problem! This will be released next week for sure. Thanks for the PR!