Add terminal interface for remote cluster connection

Proposal

Use xterm.js to create a terminal environment to remote connect to pod deployed by #418264.

Once two-way communication in gitlab-org/cluster-integration/javascript-cluster is available, we can use the exec api support added in #418265 (moved) to connect the remote shell environment to the browser UI.

This remote shell can live in the environments page, pre-configured to connect to a given cluster, or it can live within the k8s cluster management interface as a new tab Remote Access.