Split the context store interface

This is to make it easier to implement support for exporting contexts in
3rd party code, or to create mocks in tests.

2 exemples where it simplify things:
- docker-app desktop-specific context decorator (which rewrites parts of
the docker context to simplify UX when using on Docker Desktop contexts)
- ucp for including a context in the connection bundle

Signed-off-by: Simon Ferquel <simon.ferquel@docker.com>

cli/context/docker/load.go

@@ -31,7 +31,7 @@ type Endpoint struct {
 }
 
 // WithTLSData loads TLS materials for the endpoint
-func WithTLSData(s store.Store, contextName string, m EndpointMeta) (Endpoint, error) {
+func WithTLSData(s store.Reader, contextName string, m EndpointMeta) (Endpoint, error) {
 	tlsData, err := context.LoadTLSData(s, contextName, DockerEndpoint)
 	if err != nil {
 		return Endpoint{}, err