render func.now() as CURRENT_TIMESTAMP

[sjhewitt]

Please answer these questions before submitting your pull requests. Thanks!

1. What GitHub issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

   Fixes SNOW-760624: func.now() should be rendered as CURRENT_TIMESTAMP #395

2. Fill out the following pre-review checklist:

   • I am adding a new automated test(s) to verify correctness of my new code
   • I am adding new logging messages
   • I am adding new credentials
   • I am adding a new dependency

3. Please describe how your code solves the related issue.

   implements SnowflakeCompiler.visit_now_func to render func.now() as CURRENT_TIMESTAMP

[sfc-gh-sfan]

LGTM. @sfc-gh-aling please also take a look :)

I'll create a accompany PR to test.

[sfc-gh-sfan]

Test PR in #397

[sfc-gh-sfan]

Can you fix the case in your test? E AssertionError: 'SELECT CURRENT_TIMESTAMP AS now_1' != 'SELECT CURRENT_TIMESTAMP as now_1' on dialect <snowflake.sqlalchemy.snowdialect.SnowflakeDialect object at 0x7f9b46db9af0>