Create and publish Base functional layer

[mjnagel]

Additional context in #736

As accepted in the ADR we should create and publish a "base" functional layer. This layer should include Istio (and Gateways) + Pepr w/UDS Operator & UDS Policies.

Definition of done for this layer:

• New packages/base folder with a zarf.yaml named core-base importing istio, gateways, and pepr (operator + policies)
• Readme identifying the contents of the layer as well as any dependency layers (none in this case)
• Validation task file, importing validations from components
• CI filters modified to test "base" on changes to appropriate files
• CI workflows/tasks modified to add a "layer test" which builds and deploys the specified layer. This should be used on the CI filter in place of the existing single package test for the relevant package(s)
• Publishing workflows/tasks updated to publish the layer, using monolithic versioning (versioned identical to core itself) - this will also require release-please config updates

Existing draft PR may be helpful in the implementation here.