support arbitrary cloud-init metadata (aka metadata datasource values)

[diablodale]

It would be beneficial to provide an arbitrary metadata dictionary into multipass -- just like today we can pass an arbitrary userdata dictionary.

I would use this during development to simulate parts of my cloud provider's infrastructure, e.g. geographic zones, security tokens, etc.

Today, multipass hardcodes a custom metadata datasource and it provides only two values instance-id and local-hostname
https://github.com/CanonicalLtd/multipass/blob/09335ce5392897a229436b75ea81944757bc3a77/src/daemon/daemon.cpp#L102

Native cloud-init supports 17 datasources for metadata, e.g. Azure, EC2, Digital Ocean, etc. Datasources are read and the dictionaries are passed into the cloud-init pipeline where that dictionary's values are/can be used by the many cloud-init modules.

FYI: Native cloud-init already has a datasource named nocloud which somewhat aligns with this request. nocloud has only three parameters -- the same two instance-id and local-hostname plus a 3rd seedfrom which can take a file or network location.
https://cloudinit.readthedocs.io/en/latest/topics/datasources/nocloud.html