Introduce the configuration for SQL Server task databases

[morozov]

Once #9 and #12 are resolved, replace the TODO in the code with the actual logic:
https://github.com/sugarcrm/debezium/blob/efc7d20e4d973ee5e04863b5923f154ae41a7c38/debezium-connector-sqlserver/src/main/java/io/debezium/connector/sqlserver/SqlServerTaskPartition.java#L55-L56

Acceptance criteria:

1. Introduce the task.database.names configuration parameter doc using all the standards for describing the configuration (e.g. see [DBZ-2912] Add support for binary.handling.mode to the SQL Server connector debezium/debezium#2044 (comment)).
2. Replace the temporary single database name with the list of database names.
3. Add validation that the list is not empty (better if implemented at the configuration level via the framework, otherwise, the logic at the task level will suffice).