Server::send_text_owned

[maciejhirsz]

This is somewhat analogous to send_binary_mut in that it allows us to do masking in place, which is one less copy for jsonrpsee.

Doing send_text_mut would be a closer mirror, but casting &mut str into &mut [u8] is unsafe (the original str slice might end up not being valid utf8). Soketto already supports owned buffers internally though, and since jsonrpsee spits out owned Strings, we can use that to make responses that one memcpy faster.