Add registry validators (#350)

Fixes #96

## Motivation and Context

See #96

## How Has This Been Tested?

Has some unit and integration tests, still needs proper testing of the
different registry types

## Breaking Changes

Yes, new package uploads will need extra verification

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [x] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

tests/integration/docker-compose.integration-test.yml

@@ -2,6 +2,7 @@ services:
   registry:
     environment:
       - MCP_REGISTRY_SEED_FROM=
+      - MCP_REGISTRY_ENABLE_REGISTRY_VALIDATION=false
     healthcheck:
       test: ["CMD", "wget", "-qO-", "http://localhost:8080/v0/servers"]
       interval: 1s