feat: Support disk_size_gb at region config level while also preserving backwards compatibility with root level value

[AgustinBettati]

Description

Link to any related issue(s): CLOUDP-258270

Includes changes so that disk_size_gb attribute is configurable and populated in both the new location (withing electableSpecs/anayticsSpecs/readOnlySpecs) as well as the existing root location.

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 defined an isolated PR with a relevant title as it will be used in the auto-generated changelog.
• 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]

Nice!

[lantoli]

is there always an electable spec, even if not defined in the TF file?

[AgustinBettati]

Correct, the API always returns electableSpecs, readOnlySpecs, and analyticsSpecs even if not defined in the request. Example of readOnlySpecs response when not defined:

"readOnlySpecs": {
    "instanceSize": "M30",
    "diskIOPS": 120,
    "diskSizeGB": 32.0,
    "diskThroughput": 25,
    "nodeCount": 0
}

[lantoli]

it's a best practice to make sure conversion can be done, e.g. using v.(float64) with ok param

[AgustinBettati]

Synced offline. For resource using sdk v2 this is a common pattern as the schema is already ensuring a certain type for each attribute meaning this can be considered a safe cast.
Main con of handling the error is that is would add significant boilerplate for each time we access a value from the TF model. This was solved in the terraform plugin framework as we parse the TF model into a typed struct.
cc: @EspenAlbert as you were seeing these warning in the new linter.

[lantoli]

can't we use ProjectIDExecution?

[AgustinBettati]

will adjust in follow up PR, I see we have a mix of both for the advanced_cluster tests