Added MySQL System Catalogs

[terricain]

I started out looking at #1171, realising that sqlc already contains all of the Postgres catalogs, then realised we didn't have any MySQL ones. Made a copy of the sqlc-pg-gen tool for MySQL, then tweaked the codegen to exclude the right schemas from being dumped into the output.

Adds:

• Tables, Views, Functions and Procedures from the mysql, information_schema, performance_schema and sys databases.
• sqlc-dolphin-gen tool to dump the above from a MySQL cluster.

Changes:

• Schema exclusion logic to exclude specific sets of schema depending on the engine in use.

Fixes:

• Typo in sqlc-pg-gen

[terricain]

Ah looks like the schema exclusion bit in internal/codegen/golang/result.go needs to be propagated to kotlin plugin

[terricain]

Have updated the kotlin plugin here - sqlc-dev/sqlc-gen-kotlin#24