Allow adding claims to the OIDC token request

[triarius]

This adds a new claim flag that may be specified multiple times to add elements to a claims field to the JSON body of the OIDC token request to buildkite.com.

[sj26]

I tried this with the newly-available claims parameter on the agent api and it works a treat — thanks!

buildkite-agent oidc request-token --claim organization_id --claim pipeline_id | jwt decode -

...

Token claims
------------
{
...
  "organization_id": "(a correct uuid)",
  "organization_slug": "my-test-account",
  "pipeline_id": "(a correct uuid)",
  "pipeline_slug": "test",
...
}

[sj26]

💚