provider: add "validate" option that can disable validation

[bendrucker]

This allows users to disable credential validation by setting validate = false in the provider configuration.

Use Case

I recently wrote some tests for a set of complex modules that abstract details of Datadog resources, e.g. monitors and synthetic tests. While I might E2E test these configurations in a dedicated sandbox account, initially I wanted to write tests that assert some of the contents of:

terraform plan -out ./plan.out
terraform show -json ./plan.out

The resulting JSON document has planned_resources and changed_resources. There's no existing state so I only care about the planned_resources. Currently these tests require valid credentials but they would pass just the same with mock credentials if validation could be disabled.

Closes #368

[enieuw]

+1 on this, we have a use case where we conditionally create DD resources. Right now even when no DD resources are created we need to provide the provider with valid credentials so this would help us improve that!

[bendrucker]

Fixing the build, forgot to close a { 🤦‍♂️

[nmuesch]

Thanks @bendrucker for working on this! The code looks good. I left just one inline comment. Looks like there are also some conflicts based on some recent changes. Would you be able to address those?

[bendrucker]

Changes:

• Keys are now optional and validation is performed by the ConfigureFunc
• Keys must not be set if validate is false, and must be set if validate = true
• The docs explain this

Notes:

• I opted not to use ConflictsWith since that expresses that entire attributes conflict regardless of values. If a user ends up setting validate = true and providing valid keys I don't want to treat that as an error.

[nmuesch]

Left a couple last comments. (They're all really small)
Thanks again for the work here, and the docs 🙇!

[bendrucker]

Changes made!

[nmuesch]

Thanks for all the work here! Merging!