Move datafusion-flight-sql-server to a Separate Repository

[hozan23]

I believe the datafusion-flight-sql-server crate is no longer relevant to the datafusion-federation repository. The code has matured significantly and has become independent enough to moving to its own repository. It no longer has any direct connection to federation-related functionality.

[backkem]

What do you think @nathanielc? You're the only other contributor to this crate I know off.

For me splitting this off is fine. Although it may slightly increase the maintenance burden (more repo's where dependencies have to be bubbled up).