C7N Filter Refactor: Extract Mixins: Create CELFilter subclass

Problem

When using CEL filters, there will be no longer be unique Filter subclasses embedded in a policy. All unique processing for the various specialized filters need to be refactored.

Instead of specialized Filter subclasses, a new CELFilter class will be used. This must have all of the other filter subclass mixins available in it.

Examples

For two examples of the consequences of this, consider type: image-age and type: metrics filters.

• image-age. The description of an AWS EC2 instance has an image identifier, but no details on the image. The C7N ImageAge subclass of the Filter class queries image information for an EC2 instance to locate the image creation date. The processing gathers additional information not directly part of the resource description.

• metrics. A number of AWS resources have CloudWatch metrics. The C7N Metrics subclass of the Filter class queries the metrics statistics for all available resources, then applies the filter rule to all of the responses.

In both cases, a subclass of the Filter class defines the additional processing. In the case of ImageAge, it's a clearly-separated mixin. In the case of MetricsFilter, there isn't as clear a distinction between gathering data and applying the operator to the results.

When using CEL filters, there will not be a number unique Filter subclasses. Instead there will be one CELFilter subclass of Filter.

The processing must be refactored into this CELFilter object. Perhaps as plug-in strategy objects, or perhaps as mixin class definitions.

For resource types with Images, an InstanceImageBase mixin must be part of the CELFilter class.

For resource types that can provide CloudWatch Statistics, a new MetricsAccess mixin must be part of the CELFilter class.

Initially, it appears that Config, Health, KMS, Revisions, and VPC's all require refactoring of the mixins that provide raw data to the filter.

Action

Refactor the Filter class hierarchy to extract the "Data Gathering" features into easily segregated mixin classes.

Define a new CELFilter class that includes the mixins extracted from the legacy Filter subclasses.