Use sandbox proxyURL with idToken to work with sandbox cluster

[dgolovin]

Sandbox getSignupStatus request to signup service returns proxyURL among other attributes for sandbox instance. Extension should use this proxy with idToken retrieved with Red Hat Authentication extension to communicate with Sandbox instance.

[dgolovin]

Currently it responds with statusCode 500 and a message "unable to get target namespace: serviceaccounts "appstudio-${compliantUsername}" is forbidden: User "system:serviceaccount:toolchain-member-operator:toolchaincluster-member" cannot get resource "serviceaccounts" in API group "" in the namespace "${compliantUsername}""

Where ${compliantUsername} is value from json returned by getSignupStatus request.

[dgolovin]

CC: @mohitsuman