SNOW-1794377: Add support for Dataframe.toJSON

[sfc-gh-jrose]

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

   Fixes SNOW-1794377

2. Fill out the following pre-review checklist:

   • I am adding a new automated test(s) to verify correctness of my new code
     • If this test skips Local Testing mode, I'm requesting review from @snowflakedb/local-testing
   • I am adding new logging messages
   • I am adding a new telemetry message
   • I am adding new credentials
   • I am adding a new dependency
   • If this is a new feature/behavior, I'm adding the Local Testing parity changes.
   • I acknowledge that I have ensured my changes to be thread-safe. Follow the link for more information: Thread-safe Developer Guidelines

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

   This PR adds an approximation of pysparks Dataframe.toJSON method. Notably Snowpark does not support RDDs so instead we return a Dataframe. Additionally Snowflake JSON methods don't support the use_unicode flag that pyspark provides so that is left out as well. We could add it as a dummy value for compatibility, but I think we should stick to what the Snowflake API can provide.

[sfc-gh-aalam]

we also should update the dataframe.rst file

[github-actions]

Seems like your changes contain some Local Testing changes, please request review from @snowflakedb/local-testing