Feat: add ability to create dialect plugins

[vgvoleg]

Resolves: #6626

[georgesittas]

Drop a ping / bump this PR when ready.

[georgesittas]

Great work, thanks for your contribution. A few questions on my end, @tobymao you may wanna take a look at this one too.

[vgvoleg]

@georgesittas @tobymao

So there's a proof-of-concept plugin:
https://github.com/ydb-platform/ydb-sqlglot-plugin

There's entrypoints definition:
https://github.com/ydb-platform/ydb-sqlglot-plugin/blob/197203f4ef1c7ffc0d4dbe7e7c9ba6bc104c71f2/pyproject.toml#L39-L40

You can install it from github
pip install git+https://github.com/ydb-platform/ydb-sqlglot-plugin.git

And check (before and after installation) this script:

import sqlglot

query = """
with cte as (select * from b) select * from a, cte where nvl(b)>0 and EXTRACT(YEAR FROM dt) > 2024
"""

print(sqlglot.transpile(query, read="postgres", write="ydb"))