Make header forwarding configurable via proto config

`mcp_router`'s `createUpstreamHeaders()` forwards every request header to the backend except a small hardcoded skip-list (`:method`, `:path`, `:authority`, `host`, `content-type`, `accept`, the session header). That includes `authorization` and anything else the client sends.

```cpp
// mcp_router.cc:1983
// TODO(botengyao): Make header forwarding (authorization, etc.) configurable via proto config.
if (request_headers_) {
copyRequestHeaders(*request_headers_, *headers);
...
```

Since `mcp_router` aggregates multiple backends behind one filter, this matters in practice: a deployment mixing a trusted internal backend with a third-party one has no way to stop the client's `authorization` header from reaching both.

Before writing a PR, three things worth clarifying:

1. Filter-level (`McpRouter`), or per-backend (`McpBackend`)? `createUpstreamHeaders(const McpBackendConfig& backend, ...)` already takes the backend as an argument, it just doesn't consult it for header policy yet. Per-backend seems like the natural fit given mixed-trust backends are the actual motivating case, but wanted to check.
2. Reuse `config.common.mutation_rules.v3.HeaderMutation` (already embedded by `ext_authz`, `ext_proc`, `header_mutation`, `transform`), or a narrower purpose-built message like `api_key_auth`'s recent `Forwarding`?
3. Does the default behavior stay strictly additive (unset field = today's forward-everything-except-skip-list), or is the current default itself worth tightening as part of this?

Happy to put up a PR once these are settled.

Part of #39174.

cc @botengyao

Start at mcp_router.cc:1983 and inspect createUpstreamHeaders(const McpBackendConfig& backend, ...), then review the existing HeaderMutation and api_key_auth Forwarding references named in the issue. Resolve whether policy is filter-level or per-backend, select the proto representation and default behavior, and define coverage for mixed-trust backends before implementing.

Written by the indexing model from the issue text.