add references for ca resources

[tschlaepfer]

Description of your changes

This change introduces a new reference parameter for the SecretBackendRootSignIntermediate and SecretBackendIntermediateSetSigned objects which enables the creation of a CA certificate chain using the HashiCorp Vault provider.

For the SecretBackendRootSignIntermediate object, the new parameter allows to reference a SecretBackendIntermediateCertRequest object, from which the "csr" parameter is fetched.

For the SecretBackendIntermediateSetSigned object, the new parameter allows to reference a SecretBackendIntermediateSetSigned object, from which the "certificate" parameter is fetched.

The MR also includes an example of creating a CA chain using the new reference parameters.

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

See #40

[tschlaepfer]

@turkenf I did as you said and created a new branch. I ran make generate and make reviewable. The differences in the code look like they are the same.

[turkenf]

@turkenf I did as you said and created a new branch. I ran make generate and make reviewable. The differences in the code look like they are the same.

Ok, @tschlaepfer, now I am running make generate and then I will push the changes. If there are no pipeline problems, we can review your change.

[tschlaepfer]

@turkenf Thanks for fixing the code, so I guess I'm missing something (doing wrong) in my local setup. Do you have any idea what it might be?

[simonostendorf]

I am interested in this change, do you have a release timeline?

[simonostendorf]

I think it would be cool to add a backendRef option to the SecretBackendRootSignIntermediate too. This is the case for many other resources.