Ability to customize underlying GRPC connections

[zdazzy]

It is often needed to do this.

Possible scenarios:

• reporting grpc clients metrics across entire application (without resorting to agents)
• some exotic testing scenarios that require YDB to behave in a certain way (e.g. trigger TLI in a guaranteed and reproducible manner)

A possible solution: provide a jdbc property indicating a customizer class/class name that implements UnaryOperator<ManagedChannelBuilder>.

[zdazzy]

Implemented. Thank you :)