Basic validation of YAML files

[spyglass-software]

Initially, just check: Privileges are from a known list (e.g. usage, monitor, select, insert, etc.)

This helps prevent simple mistakes (e.g. typos) at the expense of requiring us to enumerate every possible Snowflake object and privilege. This is probably worth it for user experience, and we can add a apply --skip-validation flag to ignore these issues.

[spyglass-software]

Note to self: look into whether https://github.com/gristlabs/ts-interface-checker could help here