Added validation and better description for the offer_url attribute

[Ivansteinium]

Description

Added validation and better description for the offer_url attribute that the name and endpoint fields cannot be used alongside offer_url.

Fixes:
fixes #449

Type of change

<What type of a change is this? Please keep one or more relevant lines from below and delete the others.>

• Change existing resource
• Change docs

[cderici]

@Ivansteinium Thanks for the change ❤️ generally looks good, but a couple of things are needed, please:

• rebase the PR and resolve the conflicts,
• the code part seems redundant, as the validation for the mutual exclusivity of offer_url, name and endpoint are already being checked in the ValidateConfig() for integrationResource.
• fix broken tests (I assume this won't be needed after the extra validation code is removed).
• provide QA steps in PR description, if there's no code added or functionality changed, then please state that in the QA steps as well.

Thanks again! After these are done we'll take over and give it another review and land it 👍