Provide a way to set Multipass CPU and memory

[mr-cal]

What needs to get done

Provide a way for applications to define the CPU and memory for a Multipass instance.

The number of CPUs could be set by craft-application based on CRAFT_MAX_BUILD_COUNT and CRAFT_MAX_PARALLEL_BUILD_COUNT.

Alternatively, a user could define it with something like snap set snapcraft provider.cpus = <num cpus> and craft-application would pass this value to craft-providers.

I'm not sure how this will get passed to craft-providers, but it will probably be similar to how #191 will pass data about mknod needing to be enabled.

Source: #212

Why it needs to get done

Craft-providers hardcodes Multipass instances to 2 CPU cores and 2GB memory. This make builds slow (or impossible).

[syncronize-issues-to-jira]

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The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-2615.

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[lengau]

As a side note: adding too many CPUs to a VM can create its own problems, so we should be careful in that direction too