types, ast, docs: Add support for named function arg declarations

This commit adds support for named argument declarations for built-in
functions as well as additional metadata/annotations on built-in
functions (e.g., descriptions, categories, etc.) This commit allows us
to generate a data file (builtin_metadata.json) that other tools can
consume to improve the Rego authoring experience.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>

Co-authored-by: Torin Sandall <torinsandall@gmail.com>