Run proxy servers based on bundle definitions

[rhernandez35]

This commit has the following high-level changes:

• mix the PreRequest format directly into the service bundle instead of applying it at runtime in the MCP server layer
• rewrite ConfigProvider into a client plugin

Things to follow up on:

• stop mixing the PreRequest into the bundle, or at least put it into a separate "generic arguments" section so we can reuse it in other places like the CLI
• strip out the PreRequest fields from the request – right now we send the whole Document to the backend service
• nest the actual request inside the PreRequest as a document instead of mixing the fields in
• give auth schemes and identity resolvers access to the full, unredacted request context