Add SttpMockServerClient Binary Endpoint

[Husky22]

Interpret application/octet-stream as BINARY endpoint with base64 encoding.

I'm not sure whether only application/octet-stream should be interpreted as binary or also images?

[adamw]

binary vs textual is non-trivial to determine using just the content type, but I suppose if you get anything else than text/* or application/* it's binary; for application/* it might be both, so probably safer to default to text. There's also multipart, but that's yet another story