Re-add `Stop` to provider interface and differentiate stop & delete

**Describe the solution you'd like:**
With #329 we removed `Stop` from the Microvm provider interface and form the firecracker implementation as stop isn't used (we currently take action in delete).

We should add `Stop` back in and update flintlock so that the following happens:

- `Stop` stops the microvm from running (i.e. stops firecracker procvess)
- `Delete` deletes any files/resources that where created my the provider implementation. So for Firecracker it will be things like firecracker.cfg & metdata.json

**Why do you want this feature:**

**Anything else you would like to add:**
[Miscellaneous information that will assist in solving the issue.]

Start at the Microvm provider interface and the Firecracker implementation affected by #329; trace the current Delete lifecycle in flintlock. Done means Stop terminates the Firecracker process while Delete removes provider-created files and resources such as firecracker.cfg and metadata.json, with the interface and behavior aligned.

Written by the indexing model from the issue text.