Rename RegisterUpstreamMethod to RegisterActionMethod

[philbrookes]

Description

Rename RegisterUpstreamMethod to RegisterActionMethod across the gRPC proto, SDK types, server handler, and client implementation.

Acceptance Criteria

• gRPC proto RPC renamed from RegisterUpstreamMethod to RegisterActionMethod
• UpstreamConfig type renamed to ActionMethodConfig
• KuadrantCtx interface updated
• Server-side handler renamed
• Client-side implementation renamed
• Unit tests

References

• Design: extensions-SDK-action-pipeline-design.md
• Parent: Extension SDK Action Pipeline #1898

[adam-cattermole]

#1895 (comment)

Not 100% on the naming here; I know we know what this means but I think both Action and Method in RegisterActionMethod are ambiguous and I'm not sure an extension author could know what this means