Generate events from an external program using the new `exec` source

This release includes a new exec source that can be used to run programs outside of Vector to generate log events by consuming the stdout and stderr output streams. This can be especially useful to consume input from sources that Vector does not yet natively support such as querying data from a Postgres database via psql.

It is capable of either running a command on an interval or starting up a long-running command.

See the exec source documentation for more details and examples.

Thanks to @moogstuart for this contribution.