`cargo-pgx` template support

[jamessewell]

trafficstars

They replace the previous single shot bgworker config switch

All files are templated out of the ./src/templates directories

Possible to do:

• check (at compile time) that templates have a MVP list of files (./src/lib.rs, Cargo.toml etc..)
• Build (at compile time) a list of ./src/template directories to include (static atm)

[Hoverbear]

All in all I'm pretty pleased with this @jamessewell . :) In your commit you mentioned this was a first go, are you planning on making further changes or should I take it for a spin?