planner: introduce a new fix-control variable to control whether to lift plan cache restrictions temporarily

[qw4990]

Enhancement

The current plan cache has several restrictions (see https://docs.pingcap.com/tidb/stable/sql-prepared-plan-cache), and those restrictions can be resolved by changing the code, but this involves loads of work.
In some PoC or experiment environments, we want to know the theoretical best performance if all queries can hit our plan cache, so introduce a new fix-control variable to lift all plan cache restrictions temporarily for these scenarios.