chore: Convert free clusters with count

[lantoli]

Description

Convert free clusters with count

Link to any related issue(s): CLOUDP-297859

Type of change:

• Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR. A migration guide must be created or updated if the new feature will go in a major version.
• Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR. A migration guide must be created or updated.
• This change requires a documentation update
• Documentation fix/enhancement

Required Checklist:

• I have signed the MongoDB CLA
• I have read the contributing guides
• I have checked that this change does not generate any credentials and that they are NOT accidentally logged anywhere.
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code
• If changes include deprecations or removals I have added appropriate changelog entries.
• If changes include removal or addition of 3rd party GitHub actions, I updated our internal document. Reach out to the APIx Integration slack channel to get access to the internal document.

Further comments

[EspenAlbert]

I like the abstraction signature and you don't need to worry about the type of the attribute!

[EspenAlbert]

cool approach of starting with one specific case.
I guess some operations might be more generally applicable, like moving instance_size, but we'll find out once we add more cases!

[lantoli]

yes, I'll go now with one with num_shards

[EspenAlbert]

Awesome!

[lantoli]

errors file instead of having individual out files with just the error message, so it's easier to have multiple input files with errors and don't have many files, and have a place to see all error messages.