log: net: Add TCP support to syslog transmitter

Allow user to configure the syslog net backend to use TCP
instead of UDP. The syslog server address for TCP needs to
have "tcp://" URI in front of the address, for example the
tcp://192.0.2.2 server address would use TCP as a transport.
If there is no URI, then UDP would be used by default.

See the relevant RFC at https://www.rfc-editor.org/rfc/rfc6587


for details.

Fixes #66728

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>