[Minor] Migrate src/plugins/lua/clustering.lua to lua_shape

[Minor] Migrate src/plugins/lua/clustering.lua to lua_shape

Replace tableshape with lua_shape in clustering plugin.

Changes:
- ts.shape { ... } → T.table({ ... })
- ts.number, ts.string → T.number(), T.string()
- 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: rule_schema with clustering configuration including max_elts,
expire times, score multipliers for spam/junk/ham, and selectors.