Support configurable initialize sql statement for engine startup

[yaooqinn]

1. Describe the feature

Currently, the engine uses hardcoded show databases to force initialization with catalog eagerly bootstrapping too

we can make it configurable with a comma separate query list and select current_database() might be used as default since it's more lightweight than show databases in real-world cases

2. Motivation

Initialize SQLs can meet some particular cases when users may want something to be activated by default before queries.

3. Describe the solution

kyuubi.engine.initialize.sql can be introduced to achieve this goal with a default value SELECT CURRENT_DATABASE()

4. Additional context

[yaooqinn]

CC @turboFei Can you help with this?

[turboFei]

sure, with pleasure