[postgres] Support metadata columns for postgres-cdc connector

[GOODBOY008]

Expose following Postgres metadata columns in this PR:

issue: #249

Key	DataType	Description
database_name	STRING NOT NULL	Name of the database that contain the row.
collection_name	STRING NOT NULL	Name of the collection that contain the row.
op_ts	TIMESTAMP_LTZ(3) NOT NULL	It indicates the time that the change was made in the database. If the record is read from snapshot of the table instead of the change stream, the value is always 0.

[GOODBOY008]

Hi,@wuchong @leonardBang
If you are available, please help code review. Sorry for blocking meta data issue.Because of my incoming ACP exame.

[leonardBang]

Thanks @GOODBOY008 for the nice work, I'll help review this one

[leonardBang]

Thanks @GOODBOY008 for the nice work, I only left several minor comments. And we should update the documentation when we introduce new feature. Appreciate if you can also update the documentation.

[GOODBOY008]

Thanks @GOODBOY008 for the nice work, I only left several minor comments. And we should update the documentation when we introduce new feature. Appreciate if you can also update the documentation.

Thanks @leonardBang code review for my pr. I have fixed all problems.By the way,mysql connector is updated too.

[leonardBang]

Thanks @GOODBOY008 for the great work, it's cool that you also update the mysql-cdc documentation. LTGM once the CI green.