Snowflake: Add implementation for CREATE TASK statement (#1597)

[JoeHut]

Brief summary of the change made

Closes #1597

This PR adds an implementation for the CREATE TASK expression for the Snowflake dialect as described in the docs

CREATE [ OR REPLACE ] TASK [ IF NOT EXISTS ] <name>
  [ WAREHOUSE = <string> ]
  [ SCHEDULE = '{ <num> MINUTE | USING CRON <expr> <time_zone> }' ]
  [ ALLOW_OVERLAPPING_EXECUTION = TRUE | FALSE ]
  [ <session_parameter> = <value> [ , <session_parameter> = <value> ... ] ]
  [ USER_TASK_TIMEOUT_MS = <num> ]
  [ USER_TASK_MANAGED_INITIAL_WAREHOUSE_SIZE = <string> ]
  [ COPY GRANTS ]
  [ COMMENT = '<string_literal>' ]
  [ AFTER <string> ]
[ WHEN <boolean_expr> ]
AS
  <sql>

Open questions/issues:

• How to implement the WHEN clause?
• SCHEDULE clause is implemented as literal string, not broken down further
• Is the implementation of using a generic StatementSegment for the AS clause correct?

[JoeHut]

@tunetheweb, here is the PR. maybe you can help me out with the questions above.

[tunetheweb]

Looks pretty good!

Feedback below:

[tunetheweb]

BTW run this to generate the YML file from the SQL fixtures:

python test/generate_parse_fixture_yml.py

[codecov]

Codecov Report

Merging #1603 (cfbd222) into main (c61003a) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main     #1603   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          131       131           
  Lines         9180      9184    +4     
=========================================
+ Hits          9180      9184    +4     

Impacted Files	Coverage Δ
src/sqlfluff/dialects/dialect_snowflake.py	100.00% <100.00%> (ø)

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[tunetheweb]

LGTM!

This good to merge now or still working on it?

[JoeHut]

That would be good from my side. I still have some other failures, but those are unrelated so that will be a new PR in the next days probably

[tunetheweb]

Thank you! Hopefully this PR has encouraged you to (or at least hasn't discouraged you from!) adding more PRs