allow "nested fields" to be configured

[jaypipes]

When designing the pkg/generate/config.FieldConfig object, we really
only thought about (and tested for) "top-level fields". In other words,
fields that were on the CR's Spec or Status objects.

However, as we've added more configurability to the code-generator
regarding fields -- including marking fields as printable or secrets --
we're now limited substantially by our initial implementation that only
allows referring to top-level fields.

For example, we cannot mark a field as being a secret field if that
field is a subfield of one of these top-level fields. Concretely, this
means that with the existing generator configuration, we cannot support
configuring AmazonMQ's Broker CRD's Spec.Users.Password field as a
SecretKeyReference because Password is not a top-level field. It is
instead a field of a top-level field -- a "nested field" -- and
therefore our simplistic configuration system isn't able to tell the
code generator to replace the string Go type with
*ackv1alpha1.SecretKeyReference like it does for
top-level fields.

The solution to this dilemma is contained in this PR and involves
reworking our generator model in a couple ways:

1. The pkg/model.CRD struct now has a Fields attribute to go along
   with the existing SpecFields and StatusFields attributes.
   CRD.Fields is a map[string]*Field like SpecFields and
   StatusFields, but unlike those attributes, the Fields map keys
   are field paths and not the original aws-sdk-go Shape member names.
   When inspecting the AWS service API model as we do in
   pkg/generate.Generator:GetCRDs(), we now construct a field path for
   each pkg/model.Field object that corresponds to "where" in the CR the
   field is. For example, Spec.Name or Status.BrokerInstances

2. The pkg/generate.Generator has been updated with a
   processNestedFields() method to populate the new field-path-keyed
   CRD.Fields map with nested fields, making it possible to refer in our
   generator config, by field path, to fields that are not directly in the
   Status or Spec object.

A following patch will update the Generator.GetTypeDefs() method to
look up field configuration by this new field-path-based map and replace
string with *ackv1alpha1.SecretKeyReference when the field is a
secret field (Issue aws-controllers-k8s/community#743).

Issue aws-controllers-k8s/community#748

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[vijtrip2]

/lgtm

I like the distinction you made using '..' for list and Map members. Haven't seen that before.

[a-hilaly]

/lgtm

[RedbackThomson]

Q: What does this conditional shield against?

[jaypipes]

There's no need to recurse into list fields when it's a list of strings, for example. Only when it's a list of structs is it necessary to dig deeper into the nesting.