Selectors: support data types other than string from envoy attributes

[eguzki]

Currently, only selectors that return string data type can be used.

From the Envoys attributes, examples can be request.path, source.address or request.headers.<header-name>.

However, other attributes return a different type of data. Instances can be request.time, request.size, source.port or metadata based attributes.

[alexsnaps]

See #41

[alexsnaps]

see #60