several fixes for notebook instance

[modular-magician]

Fixes hashicorp/terraform-provider-google#6798. Tests are also more comprehensive now.

Release Note Template for Downstream PRs (will be copied)

notebook: fixed bug where updating certain fields would result in a no-op update call instead of a create/destroy. Now, the only field that is updatable in place is `labels`

notebook: fixed bug where many fields were being written as empty to state, causing a diff on the next plan

notebook: fixed bug where setting `network` or `subnet` to a full URL would succeed, but cause a diff on the next plan

Derived from GoogleCloudPlatform/magic-modules#3761