Add description on snapshot creation

[heldergomes]

No description provided.

[heldergomes]

because this MR is quite long, I will try to explain a little bit about some change decisions and few concerns!

storage_volume_snapshot
Despite the requirement is quite simple, the change is painful because I had to change many piece of code, therefore I decided to change only instance snapshot on this PR, but I could open a new pull request changing volume snapshot afterward.

shared/api/instance_snapshot.go
I'd expected to refactor a little bit the InstanceSnapshot struct, snapshot data such as (expiry_at and description) should be on root struct and source instance data like (instance description) should be on nested object like InstanceProperties IMHO, this would design better the model to handle better with more instance properties in the future, however this change would break API, threfore I just add SnapshotDescription field on the model.

instance_args
Handle this struct was quite challenge mostly because incus handle snapshots and instance in the same way with instance_args, so although qemu/lxc driver and shared/api/instance_snapshot I ended up handle snapshot fields such as description and expiry on root struct, I took decision to create snapshot_args as nested object of instance_args, in order to make easier add new fields on this highly used struct.

I'd rather use snapshot as a nested struct of instance on every place like driver, api, db, etc, but this change is painful, so it's what it is!

instance_snapshot_property
I tried do use the term instance_snapshot_properties, however generate_database script only knew how to create mapper like list if tables name was singular, tables with suffix ies is considered plural on the script.

I think that's all, let me know if have any question or improvements.

[heldergomes]

I will check the pipeline errors and fix it!

[heldergomes]

This PR implements #2254