Add exit processor

[artemklevtsov]

Hi,

Added exit processor. to stop process with desired status code.
Fix redpanda-data/connect#3196.

Testing:

# test.yaml
input:
  generate:
    count: 1
    mapping: root = 1

pipeline:
  processors:
    - exit: 0

output:
  stdout: {}

$  ./benthos run -s pipeline.processors.0.exit=1 test.yaml 
$ echo $?
# 1
$  ./benthos run -s pipeline.processors.0.exit=0 test.yaml 
$ echo $?
# 0

Regards.