feat: Allow adding public assets through the CLI

[Flix6x]

No description provided.

[nhoening]

I think I want a small logging entry.

[nhoening]

Maybe this could be a follow-up issue - I found that FM complains about an asset being added to an account if there is already a public asset with the same name (I added two debug logs for clarity, "New asset data:" and "Found asset:"):

± flexmeasures add asset --name Something --account-id 1 --asset-type-id 2
[FLEXMEASURES][2023-06-15 13:06:02,580] INFO: Loading config from /home/nicolas/.flexmeasures.cfg ...
Warning: Without complete redis connection settings, FlexMeasures will not be able to run forecasting and scheduling job queues.
Warning: Without complete mail settings, FlexMeasures will not be able to send mails to users, e.g. for password resets!
You might consider setting MAIL_USERNAME, MAIL_PASSWORD, FLEXMEASURES_REDIS_PASSWORD.
[FLEXMEASURES][2023-06-15 13:06:03,213] INFO: Loaded plugins: {}
New asset data: {'name': 'Something', 'latitude': None, 'longitude': None, 'generic_asset_type_id': 2, 'account_id': 1}
Found asset: <GenericAsset 37: 'Something' (wind)> with account_id: None ...
Please correct the following errors:
{'name': ['An asset with the name Something already exists in this account.']}.
 Use the --help flag to learn more.
Aborted!

I feel that the query in data/schemas/generic_assets.py (line 52) looks okay, currently I don't know why the public asset is found:

    asset = GenericAsset.query.filter(
        GenericAsset.name == data["name"]
        and GenericAsset.account_id == data["account_id"]
    ).one_or_none()