agent: Add CockpitTextStream with 'text-stream' payload type

CockpitTextStream is a channel of payload type 'text-stream'
that transfers raw textual data.

 * Only UTF-8 data is supported. There may be another binary
   stream in the future, but that's not here.
 * Only supports connecting to a unix socket for now, might
   get other connection types in the future implemented in
   the same class.

Fixes #257