Add Snowflake's Cortex LLM function as a new feedback provider + Cortex-Finetune on feedback providers

[sfc-gh-dhuang]

Items to add to release announcement:

• Heading: Add Snowflake's Cortex LLM Functions (COMPLETE) as a new feedback provider.

Note: cost tracking (token counts) work end to end but needs to figure out how to better compute overall costs for Cortex as it also incurs compute (query) costs on the data warehouses and those cost tracking is only available on Account level in Snowflake. We should put it in a separate PR.

Other details that are good to know but need not be announced:

• TODO: confirm whether we need to manually close snowflake_session with

  
  try:
  # do stuff

finally: if snowflake_session:

Close the Snowflake session

        snowflake_session.close()

![image](https://github.com/truera/trulens/assets/171001675/c70e1bdf-e6c6-4998-a74b-d3fabc73a314)

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[sfc-gh-pmardziel]

Is it still a draft?

[sfc-gh-jreini]

Please add api reference for the cortex provider & include in mkdocs.yml. See: https://github.com/truera/trulens/blob/e4813294002bff5a0fe767e5270e22524e5cb0dc/mkdocs.yml#L255