Consider supporting Identity Assertion Authorization Grant

[sberyozkin]

Description

Identity Assertion Authorization Grant, https://datatracker.ietf.org/doc/draft-ietf-oauth-identity-assertion-authz-grant/, is starting becoming more visible, not least because it is mentioned as one of the MCP Authorization extensions at https://github.com/modelcontextprotocol/ext-auth/blob/main/specification/draft/enterprise-managed-authorization.mdx

Implementation ideas

@michalvavrik This is another case that now would be possible to handle thanks to your refactoring and it is another curated ClientAssertionProvider instance. I think we need to wait for at least another iteration of that draft though.