Extend the group role map builder account alias function to accommodate other data producers

[gene1wood]

Extend the group role map builder account alias function to accommodate other data producers by only updating records in the alias map instead of overwriting the map.

This will allow another tool that connects to AWS Organizations to fetch account ID and alias information to also run to get a more comprehensive alias map (including AWS Accounts that aren't using mozilla-aws-cli)

Modify this section https://github.com/mozilla-iam/mozilla-aws-cli/blob/a6e8152dbc9efb2ed785e4b9bdb208ec09cb4cc0/cloudformation/group_role_map_builder/functions/group_role_map_builder.py#L440-L452

so that it starts by fetching the existing map file and then updating it.