Figure out how to handle Trailers

The signer currently fetches without a `TE` header so backend servers shouldn't be sending these. If we see one in spite of that, we should probably 502.

In the future, the SXG spec may end up banning chunked transfer-encoding (see WICG/webpackage#336), so it's probably not worth adding any more support than that.

Start at the signer entry point and trace how HTTP response trailers are observed when fetching from backend servers. Check the existing SXG and HTTP handling before deciding how an unexpected trailer should fail; the stated completion behavior is a 502 response, and no file or test is named.

Written by the indexing model from the issue text.