Add WIF extension based on WAG as experimental - #42

Clarified the prerequisites for the authorization server and improved the description of the message flow. Updated terminology for issuer trust and added details on token validation.

Move Pieter Kasselman's Workload Identity Federation text from
specification/draft/ to
specification/experimental/workload-identity-federation.mdx and list it in
README.md. The only changes to the text are the ones Prettier makes, so the
format check passes.

Replace the introduction, requirements and standards list with one paragraph that cites the Workload Authorization Grant draft, and map the MCP roles to the roles it defines.

Point to the draft for the typ header and claims instead of listing them. The audience becomes the authorization server's issuer identifier, and the subject an opaque identifier. Keep two MCP rules: resource is required and the grant type is advertised in metadata. Update the example to match.

Drop the flow diagram and steps, issuer key discovery, trust configuration, trust relationships, multi-tenancy, and the response and error examples.

Replace the restated considerations with references to the draft and to MCP Authorization.

Say that workload identity federation is an established pattern, that MCP
adopts it and bases it on WAG, and that this does not preclude other
mechanisms. Remove the TODO on a second assertion profile, which this
replaces. List the authors.

Co-authored-by: PieterKas <90690777+PieterKas@users.noreply.github.com>