Implement parameter binding in extended query flow

[returnString]

Postgres' extended query flow allows clients to present a statement for parsing that contains placeholder vars ($1, $2, ...$n) and later fill in these vars at the bind phase. However, sqlparser doesn't currently support placeholders (see sqlparser-rs/sqlparser-rs#145).

Once the parser supports this, we should:

• Pass the relevant data from the stream to ClientMessage::Bind (this is presently read, but immediately discarded)
• Update the portal interface to accept param bindings
• Fill out a proper ParameterDescription in response to prepared statement describe requests