Break up chat_request() into smaller generics

[hadley]

Starting with chat_openai() and backends.

[atheriel]

This appears to break chat_snowflake() at least:

> chat <- chat_snowflake()
Using model = "llama3.1-70b".
> chat$chat("Tell me a joke in the form of a SQL query.")
Error in `req_perform_connection()` at elmer/R/httr2.R:37:5:
! Failed to parse error body with method defined in
  `req_error()`.
Caused by error in `resp_body_json()` at elmer/R/provider-snowflake.R:95:32:
! Unexpected content type "NA".
• Expecting type "application/json" or suffix "json".

[atheriel]

chat_databricks() has issues as well:

> chat <- chat_databricks()
Using model = "databricks-dbrx-instruct".
> chat$chat("Tell me a joke in the form of a SQL query.")
Error in `req_perform_connection()` at elmer/R/httr2.R:37:5:
! HTTP 400 Bad Request.
• Bad request: json: unknown field "stream_options"
Show Traceback

[atheriel]

chat_azure_openai() with Entra ID credentials continues to work for me, though.

[hadley]

I have a confirmed fix for databricks, and I'm pretty sure I've identified the snowflake issue. Assuming all the tests pass, I'm going to merge this, assuming that you or I will come back to properly test snowflake prior to release.