Motivation
----------
At the moment a grpc status message is not percent-encoded which
the spec mandates. This is not a problem as long as regular printable
ascii characters are used, but other ranges mandate a percent encoding
on the wire.

Modifications
-------------
This changeset implements the percent encoding/decoding for the
grpc status message and transparently applies it in the places
where the ServiceTalk codebase would access the header previously.

The implementation is strongly aligned on the io.grpc equivalent
in order to ensure maximum compatibility across systems.

Result
------
Properly percent-encoded/decoded grpc-message payload when non
ascii-printable characters are sent.