Add unix_datagram support to syslog source

[jszwedko]

Per #4787 (comment)

Related: #4787 #5269

[emerzon]

Hello, actually we need both unix_datagram and unix_stream to be added to syslog source. The later is used by HAProxy.

[jszwedko]

Hi @emerzon ! The syslog source does already support streaming unix sockets, just not datagram.

[ollytom]

For anyone here looking to write from rsyslog to vector, you can use the socket sink with codec set to syslog:

[sources.syslog]
type = "socket"
mode = "unix_datagram"
path = "/var/lib/vector/syslog"
decoding.codec = "syslog"
socket_file_mode = 0o662

rsyslog can only write to datagram unix sockets according to the output docs.