feat: add support for overriding lang subdir

[amoeba]

What's Changed

Adds and wires through a new option to LangConfig called subdir that lets driver repos use a non-default subdirectory (or the repo root) for their driver, rather than having to put their driver code in a language-specific subdir. Having a pattern for multi-lang driver repos is good but I think supporting single-lang driver repos is worthwhile.

For example, a Go driver at the root of a repo would enable Go and set their subdir with,

driver = "something"
private = false

[lang.go]
subdir = "."

Also re-generates the generate schema and we were out of date so there are extra changes in there.