Support runas in server artifact definition

[scudette]

Normally server artifacts are collected in the user context of the launching user - this consistently applies the user's ACL to everything the artifact does which is what we want in most cases.

However we sometimes want to provide some very specialized functionality that requires elevated ACLs to low privilege users. For example this artifact allows collecting Quarantine to non admin users

name: Sudo.Quaratine
type: SERVER

parameters:
   - name: ClientId
sources:
  - query: |
      SELECT if(condition=whoami() =~ "^bob$", then=collect_client(artifacts="Windows.Remediation.Quarantine", client_id=ClientId))
      FROM scope()

This does not currently work because the artifact runs in the calling user's context and if the calling user is not an admin they can not launch the Quarantine artifact.

We need a way to either:

1. Set the access token per artifact
2. Run in the context of another user