feat(sdk): Add the plugin SDK package

[kyleconroy]

Collect a few commonly used functions and put them into one package.
Move the utils.go file from the codegen package here as well.

cc @stephen

[kyleconroy]

This was the main point of the change. The Go codegen no longer depends on the compiler or config packages.

[stephen]

Nice!

I am curious what your plan for plugins overall looks like. Is this code going to get pulled outside of internal/ later? Similarly, what other internals might get pulled out? It would be nice to let other plugins reuse parts of the go (and kotlin/python/etc) code generators.

Some other loose thoughts:

• My endgoal is thinking about how something like CRUD methods #192 could be implemented via plugin. A tricky thing with that case is if the plugin wants to know what the primary key is (currently not exposed by the compiler), or if it wants to annotate individual columns (like in the :encryption case mentioned in Extensibility / Plugin Support #288) which might be hard to support since those inline comments are not passed through in the compiler.
• From Generate Python code via a WASM plugin #1414, are you thinking plugins would all be run via wasm?

[kyleconroy]

I have a whole plan but haven’t written it up in a design doc. Will do tonight and share it here.

[kyleconroy]

braindump.doc https://docs.google.com/document/d/1DVXSINp-ZAJj9YmzKh0kTE6Cf6lFadVtH1zb9D3Dhjg/edit?usp=sharing

[kyleconroy]

It's not well documented, but the SDK package is here: https://github.com/sqlc-dev/plugin-sdk-go