Tikv: Copr SIG: Implement more UDF

[breeswish]

Feature Request

Currently there are a lot of implemented UDF in TiKV Coprocessor. However they do not have vectorized implementation and their push down is not enabled in TiDB due to lack of testing.

You can:

• Choose an UDF implementation which do not have vectorized implementation, migrate it for the vectorized framework.
• Choose an UDF vectorized implementation, add integration tests for it and enable the push down in TiKV.
• Implement a new UDF in TiKV. The implementation must have both vectorized version and non-vectorized version. Basic unit tests are required.

This task can be claimed by multiple contributors.

[hawkingrei]

I will enable the push down of JSONLength. Let me try it.

[breeswish]

Close in favor of https://github.com/tikv/tikv/issues/5751