[client.Client] Record matchers for constraints

[willbeason]

Prerequisites: #175 and #170

Now that we've copied over matching logic into frameworks, and callers explicitly call "AddConstraint" and "RemoveConstraint", we can start doing bookkeeping in local.Driver which we need to have in order to move matching from Rego to Go. For this issue, we're just keeping a copy of the matchers for each Constraint.

Add a field to local.Driver called matchers. You can probably make this a map similar to the following:

type matcherKey struct {
  Kind, Name string
}

type Driver struct {
  ...
  matchers map[matcherKey]*Matcher
}

All that needs to happen is when Constraints are added to the Driver, we unmarshal the Matcher field into the Matcher type we copied over previously. Use the bytes := json.Marshal(unstructured) and json.Unmarshal(bytes, matcher) pattern used elsewhere to conver to the Match type.

We can't actually use these matchers yet since we haven't sharded the compilation environment - but we need these ready for when we do.

[willbeason]

Decided to do this for client.Client since driver can be target-agnostic.