Add CloudFront CachePolicy managed resource

[ulucinar]

Description of your changes

This PR adds a new managed resource for the CloudFront CachePolicy API.

Depends on #782. We need to merge it first.

Fixes #236

I have:

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

How has this code been tested

I've manually tested provisioning a CloudFront CachePolicy resource/editing various fields of it/deleting it using this PR and using this PR on top of #782.

[ulucinar]

After some initial consideration on how to implement a referencing mechanism for CachePolicies in a Distribution's DefaultCacheBehavior or other (non-default) CacheBehaviors, I have raised the following feature request for ACK code-generator (if not already implemented):
aws-controllers-k8s/community#881

According to my investigation, it seems not possible atm.

Because CloudFront support is still a v1alpha1 API, as I explained in the above feature request, I'm planning to introduce an index field, which will bind a reference (either a name or a label selector or a CachePolicy ID) to a specific CacheBehavior in the cacheBehaviors array of a Distribution. And it makes sense to introduce a separate set of referencer fields for the DefaultCacheBehavior (it's also explicitly defined in Distribution, outside the cacheBehaviors array).

As also discussed in the above feature request as an alternative, we may also, for instance, ignore the cacheBehaviors field in the associated generator-config.yaml, but I would not like to maintain the complex CacheBehavior type.

[AaronME]

Thanks, @ulucinar!

This LGTM.