[Feature] Add get access token for Auth0

Currently, we have `get_access_token_for_cognito`.

https://github.com/aws/bedrock-agentcore-starter-toolkit/blob/c13214ea8ca6eb277aba9e10b6836e9e6f748a54/src/bedrock_agentcore_starter_toolkit/operations/gateway/client.py#L396-L402

We need `get_access_token_for_auth0` as well.

If you think it is helpful, I can help submit a PR.

https://docs.aws.amazon.com/bedrock-agentcore/latest/devguide/identity-idp-auth0.html

Start in src/bedrock_agentcore_starter_toolkit/operations/gateway/client.py around lines 396-402 and compare get_access_token_for_cognito with the Auth0 requirements in the linked Amazon Bedrock guide. Done means the module provides get_access_token_for_auth0 alongside the existing helper with the requested Auth0 access-token behavior.

Written by the indexing model from the issue text.