Maven VReg: Update group stats when creating/destroying cached responses

Context

In the world of Maven virtual registry MVC (API only interactions) (&14137 - closed), VirtualRegistries::Packages::Maven::CachedResponse holds a response from an upstream. This response is cached in GitLab and has a file that's stored in the Object Storage.

The cached responses creation and destruction should update the dependency_proxy_size statistic on the parent group, because cached responses are stored in the dependency proxy bucket.

What does this MR do and why?

• Update VirtualRegistries::Packages::Maven::CachedResponse model to trigger Groups::UpdateStatisticsWorker whenever a cached response is created/updated/deleted.
• Update NamespaceStatistics model to take into account the sum of the size of its cached_responses when calculating dependency_proxy_size.
• Add related specs.

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Before	After

How to set up and validate locally

Make sure the rails-background-jobs service is up & running in your GDK, and open rails console:

root_group = Group.all.detect(&:root?)

r = ::VirtualRegistries::Packages::Maven::Registry.create!(group: root_group)
u = ::VirtualRegistries::Packages::Maven::Upstream.create!(group: root_group, url: 'https://repo1.maven.org/maven2')
VirtualRegistries::Packages::Maven::RegistryUpstream.create!(group: root_group, registry: r, upstream: u)

# create a cached response
cr = VirtualRegistries::Packages::Maven::CachedResponse.create!(group: root_group, upstream: u, size: 2398, file: Tempfile.new('33'), relative_path: "/path/to/file.txt")

# check the `namespace_statistics` of the group, the `dependency_proxy_size` attribute should be 2398 or more (if there are any other depencdency proxy files included in the calculation)
root_group.namespace_statistics

# check the `root_storage_statistics` of the group, also the `dependency_proxy_size` attribute should be updated
root_group.root_storage_statistics

# delete the cached_reponse and then check if the stats is updated
cr.destroy!
root_group.namespace_statistics
root_group.root_storage_statistics

Related to #482493 (closed)