bug: using alter source add columns on a table with conn will panic.

[Rossil2012]

Describe the bug

Described in the title. I suggest to reject the statement when user applies alter source on a table with conn.
cc @tabVersion.

Error message/log

ERROR:  Panicked when handling the request: should contains only one statement: ExactlyOneError[RemainingIter: IntoIter([])]
This is a bug. We would appreciate a bug report at:
  https://github.com/risingwavelabs/risingwave/issues/new?labels=type%2Fbug&template=bug_report.yml
server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.
The connection to the server was lost. Attempting reset: Succeeded.

To Reproduce

CREATE TABLE src (v INT) WITH (
    connector = 'datagen',
    fields.v.kind = 'sequence',
    fields.v.start = '1',
    fields.v.end  = '10',
    datagen.rows.per.second='15',
    datagen.split.num = '1'
) FORMAT PLAIN ENCODE JSON;
ALTER SOURCE src ADD COLUMN b int;

Expected behavior

Rejected since the source is created with create table.

How did you deploy RisingWave?

./risedev d

The version of RisingWave

PostgreSQL 9.5.0-RisingWave-1.7.0-alpha (unknown)

Additional context

No response