Add support for in-memory deployment (MAAS >= 3.5)

[vasartori]

We would like to request support for in-memory deployment in the MAAS CAPI provider.

MAAS has supported deploying machines directly in memory since version 3.5, and this feature is important for scenarios involving ephemeral or high-memory workloads, where disk usage during deployment should be minimized.

Proposed Change:
Introduce a configuration option (flag) to enable in-memory (false as default) deployment in both:

• MAASMachineTemplate
• MAASMachine

This flag would allow users to explicitly request that machines are deployed using MAAS’ in-memory deployment mechanism.

Technical Details:
To make this work correctly, the MAAS API client needs to send the corresponding deploy-in-memory field when triggering a deployment.

I have already opened a PR to add this support to maas-client-go: spectrocloud/maas-client-go#39

Once this change is merged and released in maas-client-go, I can submit a follow-up PR to the MAAS CAPI provider implementing the in-memory deployment support using the new flag.