make `enable_distsql` defaults on

tablet server uses this flag `enable_distsql` to enable cluster optimization https://github.com/4paradigm/OpenMLDB/blob/b685e648e4467d86ce1ad8233b9428fb50313e09/src/tablet/tablet_impl.cc#L210-L214. It is on with release configurations: `release/conf/tablet.flags.template`, but no in `src/flags.cc`. Which as a results, won't enabled in mini_cluster tests, e.g `sql_sdk_test`.

There is a PR previously #3295, but turns out breaks existing tests. It means there exists cases that is OK with request mode, but not request(cluster) mode.

#3531 is a example, deployment tests not fail for those tests, simply because `enable_distsql` not turned on.

The flag `enable_distsql` is defined in src/flags.cc and used in src/tablet/tablet_impl.cc lines 210-214. Check release/conf/tablet.flags.template for its default in release builds. Run mini_cluster tests like sql_sdk_test to see failures when the flag is enabled, as indicated by previous PR #3295. Investigate why certain tests pass in request mode but fail in request(cluster) mode, referencing issue #3531 as an example.

Written by the indexing model from the issue text.