[Kubernetes MCPRegistry] Conversions from and to upstream format

[jhrozek]

The idea is to support the upstream format in the API. We should aim to use the upstream types and the conversion utilities in the toolhive-registry project as they seem to be quite reusable.

Deliverable

• The converter methods for going back and forth between the ToolHive format and the Upstream format
• Initial implemention in ToolHive for using upstream-compliant registries, i.e. ones that implement the reference API spec
• no hand-rolled conversions, aim for code reuse. If we find the conversions lacking, we contribute there instead

Disclaimer: The goal is to have the converters living in toolhive-registry. For the sake of moving forward with the implementation we'll have the converters living inside toolhive and then reorganise the packages in the way we see best.

[dmartinol]

completed by stacklok/toolhive-registry-server#76