Add support for CloudSearch

[mhoff]

Description of your changes

Implements basic support for CloudSearch domains.
Some implementation details still up for discussion.

Fixes #1007.

I have:

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

I will:

• Write tests for the code in setup
• Add examples

How has this code been tested

Simple create/delete/update procedures with test resources.
End-to-end testing from creating a domain up to using it to query data using curl and anonymous access (index fields not covered by this PR).

Discussion

The AWS API for CloudSearch does provide a lean data model for Domains (name, region, current scaling values, endpoints). Other parameters (like scaling options, access policy, availability options, index fields) are handled through separate Define/Describe functions, which therefore are excluded from the generated domain data model.
In this PR, I chose the approach to include some of those parameters in the custom domain parameters and to use (sometimes contextless) sub-calls to handle late-init, isUpToDate and update.

[MisterMX]

Thanks @mhoff for your contribution! Good first PR.

I added some remarks to your code.

[mhoff]

Thanks @mhoff for your contribution! Good first PR.

I added some remarks to your code.

Many thanks for your remarks! I have resolved the few minor ones and will continue with the rest.

[mhoff]

@haarchri I have updated the PR according to what we have discussed. This PR should properly implement alpha-support for AWS CloudSearch. Review would be very much appreciated.

[haarchri]

NAME                                                           READY   SYNCED   EXTERNAL-NAME
domain.cloudsearch.aws.crossplane.io/cloudsearch-test-anonym   True    True     cloudsearch-test-anonym
domain.cloudsearch.aws.crossplane.io/cloudsearch-test-domain   True    True     cloudsearch-test-domain

[haarchri]

LGTM @mhoff thanks for contribution and all changes & rebases