Expose GetCapabilities for validationHandler in app

## Description
[eFormidling]
In some cases validating the capabilities of the orgno can be useful. The client library should expose this capability and it should be possible for an app developer to use this in ValidationHandler.

It will be optional for the app developer to enable this. A benefit of this is giving a reasonable error to the end-user before they attempt sending in the data and then have this process fail.

## Acceptance criteria
> Describe criteria here (i.e. What is allowed/not allowed (negative tesing), validations, error messages and warnings etc.)

## Specification tasks

- [ ] Development tasks are defined

## Development tasks
> Add tasks here

## Test
> Add test cases here as checkboxes that are being tested as part of the changes.

## Definition of done
Verify that this issue meets [DoD](https://confluence.brreg.no/display/T3KP/Definition+of+Done#DefinitionofDone-DoD%E2%80%93utvikling) (Only for project members) before closing.

- [ ] Documentation is updated (if relevant)
- [ ] Technical documentation (docs.altinn.studio)
- [ ] User documentation (altinn.github.io/docs)
- [ ] QA
- [ ] Manual test is complete (if relevant)
- [ ] Automated test is implemented (if relevant)
- [ ] All tasks in this userstory are closed (i.e. remaining tasks are moved to other user stories or marked obsolete)

Start by locating the ValidationHandler entry point and the client-library capability APIs related to GetCapabilities. The issue does not name files, tests, validation rules, or error wording, so those requirements need to be defined before implementation. Done should mean an app can optionally validate orgno capabilities in ValidationHandler and report a useful failure before sending data.

Written by the indexing model from the issue text.