Draft: Support for `initial_replication_key_value` in meltano.yml

Support for initial_replication_key_value in meltano.yml config. Fixes meltano#2734 (closed)

Works similarly to start_date but on a per-stream basis and also handles non-date replication keys.

To use, set initial_replication_key_value alongside replication_key in the metadata section of meltano.yml. This value will be used as starting point for replication for situations where a table is already partially replicated or only partial replication is desired. Will only be used on initial sync, i.e. when there is no replication_key for the stream in the state.

TODO: Update Docs