feat: add sample for converting connection auth to Snowflake keypair

[jacalata]

Summary

Adds samples/update_connection_to_keypair.py, a runnable end-to-end sample showing how to convert an existing Tableau Cloud datasource or workbook connection from username/password auth to Snowflake keypair auth.

• Takes the item LUID and target Snowflake account username as arguments.
• Optionally targets a specific connection LUID; otherwise finds and uses the first Snowflake connection on the item.
• Uses the existing ConnectionItem fields (auth_type, username, embed_password) already serialized correctly by Connection.update_req — no library changes required.
• Re-fetches the connection after update and prints the new auth_type for confirmation.
• Prominently calls out the prerequisite that the private key must first be saved on the site under Site Settings -> Saved Credentials for Data Sources.

Supported since REST API v3.27.

Closes #1602 (paired with the docs PR against gh-pages).

Test plan

• python samples/update_connection_to_keypair.py --help shows expected args
• Manual: run against a Tableau Cloud site with a pre-saved Snowflake private key credential and verify the connection's auth_type flips to auth-keypair and extract refresh succeeds
• Manual: run without pre-saved credentials and confirm the printed reminder is visible

Refs #1602

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