Let boolean settings be passed to tap/target as stringified `'true'` or `'false'`
For some reason, https://github.com/transferwise/pipelinewise-target-postgres (#2425 (closed)) requires the ssl
setting to have a value of 'true'
(string) to enable SSL, instead of true
(boolean): https://github.com/transferwise/pipelinewise-target-postgres/blob/e7eb0c6edc253d2e98dbbd56932091492e883867/target_postgres/db_sync.py#L302
In Meltano we should just treat the setting as kind: boolean
and convert the value at the last minute.