[FEATURE] Add Flint config for mandatory checkpoint location option

[dai-chen]

Is your feature request related to a problem?

Currently checkpoint location option is optional or mandatory based on Spark stream requirement. In some case, Flint library user may want to make checkpoint mandatory for strong resilience.

What solution would you like?

Add a new Flint configuration spark.flint.index.checkpoint.mandatory with default true.

What alternatives have you considered?

Make checkpoint location option required without flexibility.