[Minor] Migrate lualib/plugins/ratelimit.lua to lua_shape

[Minor] Migrate lualib/plugins/ratelimit.lua to lua_shape

Replace tableshape with lua_shape in ratelimit utilities.

Changes:
- ts.shape { ... } → T.table({ ... })
- ts.number + ts.string / fn → T.one_of({T.number(), T.transform(T.string(), fn)})
- :is_optional() → :optional()
- Added comprehensive documentation to all fields

Schema: bucket_schema for ratelimit bucket with burst/rate limits,
optional symbol, message, and skip flags.