refactoring: extract public api to module and add docs

src/api.rs

src/api/calculate_bucket_id.rs

src/api/calculate_bucket_id_by_dict.rs

src/api/exec_query.rs

src/api/helper.rs

src/api/invalidate_cached_schema.rs

src/api/load_lua_extra_function.rs

src/executor/result.rs

src/lib.rs

src/parser.rs

src/parser/extargs.rs

src/parser/extargs/tests.rs

test_app/README.md

test_app/app/roles/api.lua