KNOX-3273 - Short Lived Tokens as Client Credentials

[lmccay]

KNOX-3273 - Short Lived Tokens for Client Credentials Flow

What changes were proposed in this pull request?

This change will extend the client credentials flow support to include the use of
JWT tokens rather than long lived client_id and client_secret.

This is preferred for scenarios where short lived JWTs are readily available to clients
such as Service Accounts within k8s clusters and projected JWT credentials.

Rather than using client_id and client_secret as bearer or HTTP basic credentials,
we will use the client_assertion param based on the client_assertion_type of
"urn:ietf:params:oauth:client-assertion-type:jwt-bearer".

POST /token.oauth2 HTTP/1.1
Content-Type: application/x-www-form-urlencoded

grant_type=client_credentials&
client_assertion_type=urn:ietf:params:oauth:client-assertion-type:jwt-bearer&
client_assertion=eyJhbGciOiJSUzI1NiJ9... <- K8s SA JWT
scope=openid profile email

How was this patch tested?

Existing and new unit tests were run and written.

[github-actions]

Test Results

7 tests   7 ✅  1s ⏱️
1 suites  0 💤
1 files    0 ❌

Results for commit 596a71f.