Merge conformance tests into crossplane/crossplane?

### What problem are you facing?

These conformance tests are effectively integration tests. We intend to version them alongside Crossplane releases, i.e. there will be a 1.2 branch of this repository to correspond to the release-1.2 branch of the crossplane/crossplane repository.

Merging these tests into crossplane/crossplane (e.g. under a `conformance/` directory) would enable Crossplane contributors to update the conformance tests in the same pull request they open to add a new feature. It would also potentially allow us to use the conformance test suite as a general integration test suite.

### How could conformance help solve your problem?

We could deprecate this repository and move the conformance tests into crossplane/crossplane. We should also assess this repository as it relates to crossplane/test, which could potentially replace this repository in future.