Make schema queries available with pool

[MikhailNazarov]

It's required to use schema executor for schema queries:

let conn = pool.acquire().await?;
    sqlx::query("CREATE TABLE test2 (id Uint64 NOT NULL, name Utf8, age UInt8, description Utf8, PRIMARY KEY (id))")
        .execute(conn.schema())
        .await?;

Now it's available from connection object. We need provide more convenient way get schema executor from db connection pool

[MikhailNazarov]

We can now use extension method schema for YdbPool

sqlx::query("CREATE TABLE test2 (id Uint64 NOT NULL, name Utf8, age UInt8, description Utf8, PRIMARY KEY (id))")
        .execute(pool.schema())
        .await?;