[enhancement]: Support passing assertions URL and/or store id

[mastier]

Enhancement

Currently the support for snap is very limited in 2.53 (link)

#cloud-config

snap:
  assertions:
    00: |
      signed_assertion_blob_here
    02: |
      signed_assertion_blob_here
  commands:
    00: snap create-user --sudoer --known <snap-user>@mydomain.com
    01: snap install canonical-livepatch
    02: canonical-livepatch enable <AUTH_TOKEN>

Normally would be safer to just pass assertions url of snap-store-proxy for for airgapped environment

Currently I use this

#cloud-config
snap:
  commands:
    00: bash -c 'snap ack <(curl -s https://snap-store-proxy.local/v2/auth/store/assertions
)'
    01: snap set core proxy.store=\$(curl -s https://snap-store-proxy.local/v2/auth/store/assertions
 |grep "^store:"| cut -f2 -d" ")

and that works by the way, as in my case the certificate for snap-store-proxy is signed by global authority (SectiGo). In other case we would need set also CA certificate.

When that is implemented the MAAS could implement setting snap-store-proxy easily for newly deployed airgapped nodes