Add template_searchpath support

[tiger24l7]

Hi! The problem is that ClickhouseOperator doesn't support template_searchpath dag option which allows to pass filename instead of full query

Instead of:

ClickHouseOperator(
    task_id='task_1',
    clickhouse_conn_id='clickhouse_default',
    sql='SELECT * from db.some_table;' # <-- raw SQL query
)

We can write:

ClickHouseOperator(
    task_id='task_1',
    clickhouse_conn_id='clickhouse_default',
    sql='select_query.sql' # <-- filename of jinja template which is located in template_searchpath location
)

One more example how it works

[tiger24l7]

Look like it's already done https://github.com/whisklabs/airflow-clickhouse-plugin/pull/41 But requires some additional things

[bryzgaloff]

Hi @tiger24l7, thank you for you feedback. Yes, there is an open PR, you are welcome to contribute :)

[tiger24l7]

My python skills are poor I might try to implement this later But currently, would be nice if someone implement this, if possible :)

[bryzgaloff]

Contribution is welcome, thank you! I will review you PR and let you know how you may enhance the code.

[bryzgaloff]

Implemented in #62