Support skip indexes

[stephen-up]

Hi, it would be great if dbt-ch supported skip indexes, much like it does projections.

Eg

{{ config(  
  engine='MergeTree()',
  order_by='id,
  materialized='table',
  indexes=[{
    'name': 'minmax_received_ts',
    'column': 'received_ts',
    'type': 'minmax',
    'granularity': '2'
  }],
) }}

That would run statements like

ALTER TABLE {{ this }} ADD INDEX IF NOT EXISTS {{ name }} {{ column }} TYPE {{ type }} GRANULARITY {{ granularity }};
ALTER TABLE {{ this }} MATERIALIZE INDEX {{ name }};

Currently, I have a work around, and define them in post_hooks but it times out sometimes.

Thanks

Is your feature request related to a problem? Please describe.
We have trouble with long run times, poor performance and stability.

Describe the solution you'd like
It would be great if dbt supported skip indexes, like it does projections.

[stephen-up]

Added in #397