Support persist docs on Snowflake: Relations and columns

[drewbanin]

Child issue of #1573

Notes:

• We can render multiline comments using $$ string quoting

To do:

• Support persisting docs for relations (tables and views)
• Support persisting docs for columns in tables and views

If the project specifies a column that does not exist in the destination table, the comment on column statement may fail. This is the desired behavior.

Challenges:

• Snowflake does not support adding comments to views.... leave out support for view comments for the time being, but consider something like strict field validation for schema.yml #1570 to allow us to generate DDL for the view with comments included at build-time.

[drewbanin]

closed by #2321