feat/add diag validator example

[mavogel]

Additions

• adds an example for validators
• adds a test make target
• see feat: add example of usage of ValidateDiagFunc #47

Missing steps

• I cannot add the github.com/hashicorp/go-cty to `go.mod due to the error

github.com/google/martian/[email protected]: verifying module: checksum mismatch
        downloaded: h1:+rpPnUc12J3godXW5lHIevv9/7Z8RjnTR3D0pyFtRPc=
        sum.golang.org: h1:pMen7vLs8nvgEYhywH3KDWJIJTeEr2ULsVWHWYHQyBs=

SECURITY ERROR
This download does NOT match the one reported by the checksum server.
The bits may have been replaced on the origin server, or an attacker may
have intercepted the download attempt.

For more information, see 'go help module-auth'.

Do you folks have any idea? Maybe #57 will fix it

Update: fixed it with go clean -modcache && go mod tidy

[hashicorp-cla]

All committers have signed the CLA.

[bflad]

Hi @mavogel 👋 Thank you for raising this.

Having appropriate documentation on ValidateDiagFunc would be great, although it feels like this would be better suited where the existing validation documentation lives, rather than adding code to the scaffolding repository here. The scaffolding code is not intended to be fully exhaustive of all SDK functionality since attempting to do so may introduce confusion for newer provider developers. We intend more advanced usage, such as writing custom validation functions, to be discoverable via the Terraform documentation.

Thanks again for the submission though!