feat: add the service instance endpoint

[lholmquist]

fixes #121

@tremes want to give this a try

the below example should do something

'use strict';

const openshiftRestClient = require('openshift-rest-client').OpenshiftClient;

(async function () {
  const client = await openshiftRestClient();

  const sc = await client.apis['servicecatalog.k8s.io'].v1beta1.serviceinstance.get();

  console.log(sc);
})();

[coveralls]

Pull Request Test Coverage Report for Build 374

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 373:	0.0%
Covered Lines:	43
Relevant Lines:	43

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💛 - Coveralls

[tremes]

Works fine for me. Even e.g await client.apis['servicecatalog.k8s.io'].v1beta1.ns("evals11-shared-9ef0").serviceinstance.get(). Thanks Luke.