Draft: Support for `initial_replication_key_value` in meltano.yml config. Fixes #2734

Allows a initial_replication_key_value to be set on a per-stream basis. This setting is only honored on the first sync, when no previous state exists. The user can set this setting similar to start_date, but on a per-stream basis and for both date and non-date based replication_keys. To use, set initial_replication_key_value alongside your replication_key in the metadata section of meltano.yml.

TODO: Update documentation