Flink: Use SerializableTable in FlinkInputFormat

[aokolnychyi]

This PR migrates FlinkInputFormat to use SerializableTable instead of TableLoader to support changes in #2984.

Fixes #2978.

[aokolnychyi]

cc @stevenzwu @openinx @chenjunjiedada @JingsongLi

[stevenzwu]

LGTM. FlinkSource has two execution modes:

1. batch. A read-only table should be fine for FlinkInputFormat.
2. streaming execution. there is a separate StreamingMonitorFunction class that takes in TableLoader. So it is not affected by this change.

Since this is a change in a critical section, I would also wait for @openinx or @JingsongLi to take a look.

[aokolnychyi]

Thanks for taking a look, @stevenzwu!

Since this is a change in a critical section, I would also wait for @openinx or @JingsongLi to take a look.

Definitely agree!

@openinx @JingsongLi Like I said before, I can also keep the old behavior if needed and just pass one more argument through all these classes in #2984. Using a table seems cleaner but not a hard requirement. Up to you.

[aokolnychyi]

@openinx @JingsongLi, could you check this PR whenever you get a sec? If it makes sense, I'll rebase.

[rdblue]

@aokolnychyi, can you rebase this? I'll review.

[aokolnychyi]

I'll close this for now. We can come back to this once we want to support _partition metadata columns in Flink.