Add partial config file functionality to cEOS

[CapnCheapo]

Added partial config support for cEOS modeled after how it's implemented in vIOS and other node types.

[hellt]

/cc @ankudinov

[ankudinov]

@CapnCheapo Thank you for submitting the PR.
Could you please add following to this PR:

1. Documentation explaining how partial config works. Similar to how it's done for other node types where partial config is supported.
2. Provide steps to test this PR before merging to reduce time required for reviewers to build the lab. @hellt I'd suggest to that as a requirement to Containerlab PR templates.

I'd appreciate if you provide a few use cases where this feature is critical. The motivation is clear in general, but I have a very strong opinion that Containerlab users should always provide a full startup config for every node instead of relying on defaults. This helps to avoid confusion in case of changes on EOS or Containerlab side and guarantees clean start of the lab. However I have a very specific workflow in mind where the full state of the lab is dictated by the Git repo. I imagine that for an average user that can be a useful feature. But would be great to see a few specific use cases where it's a must in the docs. With possible risks highlighted.