Closes #17791

This PR adds ability to cancel workspace builds that are in "pending" status.

Breaking changes:

• CancelWorkspaceBuild method in codersdk now accepts an optional request parameter

API:

• Added expect_status query parameter to the cancel workspace build endpoint
• This parameter ensures the job hasn't changed state before canceling
• API returns 412 Precondition Failed if the job is not in the expected status
• Valid values: running or pending
• Wrapped the entire cancel method in a database transaction

UI:

• Added confirmation dialog to the Cancel button, since it's a destructive operation
• Enabled cancel action for pending workspaces (expect_status=pending is sent if workspace is in pending status)