docs: proposal for OAuth 2.0 Token Exchange as Upstream Auth for MCP Backends (#2052)

**Description**

This PR adds a proposal to implement OAuth 2.0 Token Exchange as
Upstream Auth for MCP Backends. The proposal focuses on the APIs to
enable token exchange for selected backends.

**Related Issues/PRs (if applicable)**

Proposal for #2036

**Special notes for reviewers (if applicable)**

N/A

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Signed-off-by: Ignasi Barrera <nacx@apache.org>

Author: nacx