feat: add vrack data source

[mantasak]

Description

Adding single vRack retrieval data source support

There is currently no raised issue that needs this functionality, however I have noticed it is missing and I need it for a project.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Simple unit test like implemented for VPS retrieval.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings or issues
• I have added acceptance tests that prove my fix is effective or that my feature works
• New and existing acceptance tests pass locally with my changes
• I ran succesfully go mod vendor if I added or modify go.mod file

[amstuta]

Thanks for your contribution @mantasak !

Apart from the comments I left, can you also add a documentation page in website/docs/d/ ?

[amstuta]

Suggested change

	serviceName,
	url.PathEscape(serviceName),

[amstuta]

could you move this definition in the test func itself to avoid the global variable ?

[amstuta]

can be removed, there is already the testAccPreCheckVRack func that does the same thing