Support "trusted profile assume" with a go authenticator.

[sjhx]

Is your feature request related to a problem? Please describe.
missing feature I believe at least in the go sdk

Describe the solution you'd like
Essentially do this:

curl -X POST "https://iam.cloud.ibm.com/identity/token"\
  --header "Content-Type: application/x-www-form-urlencoded"\
  --data-urlencode 'grant_type=urn:ibm:params:oauth:grant-type:assume'\
  --data-urlencode 'access_token=ACCESS-TOKEN'\
  --data-urlencode 'profile_id=ProfileId-123123123'

Describe alternatives you've considered
unable to see how to do this with the SDK publisjed

Additional context
I could not find a way with the provided SDK to assume a trusted profile.
I ended up writing an authenticator to wrap and IAMAuthenticator (containing service apikey) which generates a token for the trusted profile that the apikey is allowed to assume. I think this should be provided.

[padamstx]

Looking into this... will consult with IAM team for more info.