Extend Accept-Signature to include app/s-e response type

[twifkak]

The Accept-Signature header says:

When a server receives an Accept-Signature header field in a client request, it SHOULD reply with any available Signature header fields for its response that the Accept-Signature header field indicates the client supports.

However, the same-origin Signature response may be less reliable than application/signed-exchange, due to possible undesired transformations of signed data (headers or payload). Consider modifying this section to say that a server MAY reply with an application/signed-exchange instead.

[jyasskin]

That sounds like a good change.