HTTP2/GRPC spec tweaks

[mstksg]

Summary:

• http authority can be different from destination
• no space before timeout unit -- this causes issues in the go grpc server

Issues:

1. Not sure what the best UX would be for specifying the authority -- Maybe String seems appropriate since it's expected to just be a literal header value? But it's also not obvious what's going on if you aren't already familiar with http2/grpc.
2. Not sure if we want to also provide this ability to the TLS version, but I'm also not totally sure that that is supported in http2 in any case.

[edsko]

So just for reference, grpcurl takes an address as argument, documented as

The address will typically be in the form "host:port" where host can be an IP
address or a hostname and port is a numeric port or service name. If an IPv6
address is given, it must be surrounded by brackets, like "[2001:db8::1]". For
Unix variants, if a -unix=true flag is present, then the address must be the
path to the domain socket.

and then accepts an optional -authority argument:

  -authority string
        The authoritative name of the remote server. This value is passed as the
        value of the ":authority" pseudo-header in the HTTP/2 protocol. When TLS
        is used, this will also be used as the server name when verifying the
        server's certificate. It defaults to the address that is provided in the
        positional arguments.

[edsko]

Superseded by #46, which includes the work from this PR, cleans it up a bit, and extends it to TLS also.