Add support for a method allowlist

[ethanpailes]

pggen generates a lot of code, and it is better to avoid bloating executables or stressing the compilers too much.

The [[table]] toml table should grow a new config option methods which contains an optional whitelist of methods for pggen to generate so that people can avoid generating quite so much code.

[ethanpailes]

Maybe there should also be a global config option allowing people to set a default_methods list for the whole config. They could set it to the empty list to ask for no methods to be generated.