Add validation to provider fields in both SDK and PF implementations of provider schema

[modular-magician]

Description

Closes hashicorp/terraform-provider-google#14447

This PR adds explicit validation feedback to users, to accompany the changes in GoogleCloudPlatform/magic-modules#9014

The PR above means that "" values will be processed by all the usual provider configuration logic but the error messages returned to users may be confusing and result in GitHub issues being opened. By adding explicit validation users will be able to identify and address the problems themselves.

I've chosen to add empty string validation to the more popular fields and avoiding things like request_reason because I don't know if there's a valid use case for setting that value to an empty string or not?

Testing

I've added:

• unit tests for the new empty string validators
• acceptance tests showing the validators are present in the provider config and throw errors in appropriate scenarios

Here's a screenshot from a manual test. NOTE: there are always 2 errors, as both the SDK and PF validators are rejecting the bad input

Misc details

Previously I thought that provider-level validation protecting against empty strings was a problem due to the SDK being weird/making it easy to footgun when handling zero values. After some manual tests I found this wasn't the case, so added anti-empty string validation to the SDK and PF versions of the provider config code.

NOTE: If validation exists on one version of the provider config but not the other then there's a fundamental error in the provider, as provider schemas must match when muxed together.

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Release Note Template for Downstream PRs (will be copied)

provider: added provider-level validation so these fields are not set as empty strings in a user's config: `credentials`, `access_token`, `impersonate_service_account`, `project`, `billing_project`, `region`, `zone`

Derived from GoogleCloudPlatform/magic-modules#9050