Enterprise managed authorization

[radar07]

auth, oauthex: implement Enterprise Managed Authorization (SEP-990)
This PR implements Enterprise Managed Authorization (SEP-990) for the Go MCP SDK, enabling MCP Clients and Servers to leverage enterprise Identity Providers for seamless authorization without requiring users to authenticate separately to each MCP Server.
Overview
Enterprise Managed Authorization follows the Identity Assertion Authorization Grant specification (draft-ietf-oauth-identity-assertion-authz-grant), implementing a three-step flow:

1. Single Sign-On (SSO): User authenticates to the MCP Client via enterprise IdP (Okta, Auth0, Azure AD, etc.)
2. Token Exchange (RFC 8693): Client exchanges ID Token for Identity Assertion JWT Authorization Grant (ID-JAG) at the IdP
3. JWT Bearer Grant (RFC 7523): Client exchanges ID-JAG for Access Token at the MCP Server
   This enables:

• For end users: Single sign-on across MCP Clients and Servers—no manual connection/authorization per server
• For enterprise admins: Centralized visibility and control over which MCP Servers can be used within the organization
• For MCP clients: Automatic token acquisition without user interaction for each server

[maciej-kisiel]

Hi @radar07, thanks for submitting this PR. Could you link the issue that it is addressing?

Also, as a heads-up: it will likely take some time to review your proposal. Both because it's quite large, but more importantly I'm also working on a proposal how to structure the client-side OAuth implementation and this change will need to be aligned with it.