JDBC Connection pool support for Pulsar JDBC sink connectors

**Is your feature request related to a problem? Please describe.**
Hi, Team,

Recently I want to try Pulsar [CDC Source Connector](https://pulsar.apache.org/docs/next/io-cdc) and [JDBC Sink Connector for PGSQL](https://pulsar.apache.org/docs/next/io-quickstart#connect-pulsar-to-postgresql) to sink multiple table data between PGSQLs.

But I find that the [JDBC sink connector for PGSQL](https://pulsar.apache.org/docs/next/io-jdbc-sink/#example-for-postgresql) is configured per-target-table basis; it seems that I can not sink several different target tables in a config file.

Although I can set up multiple configuration files for each target tables (the same target PGSQL database), this might cause the target side PGSQL pool performance due to many occupied connections.

In PGSQL, it is always a good practice [to utilize connect pool when many client-side connections are needed.](https://wiki.postgresql.org/wiki/Number_Of_Database_Connections#The_Need_for_an_External_Pool); and this might also holds for other databases (e.g. Cassandra).

I think Pulsar can provide JDBC connection pool for all supported JDBC sink connectors and other sink connectors if possible.
https://jdbc.postgresql.org/documentation/head/datasource.html
https://docs.datastax.com/en/developer/java-driver/4.14/manual/core/pooling/
https://dev.mysql.com/doc/connector-j/8.0/en/connector-j-usagenotes-j2ee-concepts-connection-pooling.html
https://mariadb.com/docs/connect/programming-languages/java/connection-pools/

**Describe the solution you'd like**
If my understanding is correct, Pulsar IO connectors are controlled by [`pulsar-admin` tool](https://pulsar.apache.org/docs/next/io-use/), it would be great to create connection pool for specified connector with some option of `pulsar-admin sinks` command.

- Providing a subcommand `pulsar-admin sinks pool` to create a connection pool resource
- `pulsar-admin sinks pool` may provide `--name` and `--pool-config`, etc, just like [`pulsar-admin sinks create` suboption.](https://pulsar.apache.org/tools/pulsar-admin/2.11.0-SNAPSHOT/#-em-create-em--30)
- Providing a new [JDBC sink configuration property](https://pulsar.apache.org/docs/next/io-jdbc-sink/#configuration) `connpool` to link with pre-configured connection pool.

**Additional context**
As far as I know, in kafka connect there is no such feature available.
If Pulsar can provide such database performance-friendly feature, this feature can be attractive for new users.

Best Regards.

Start by reading the pulsar-admin sinks create interface and the JDBC sink configuration documentation linked in the issue. Define how a pool resource and the connpool property would work across supported JDBC sink connectors, then verify that the requested pool configuration can support multiple target tables and connectors.

Written by the indexing model from the issue text.