chore: Updates user agent to include if PreviewProviderV2AdvancedCluster env var is enabled

[maastha]

Description

Updates user agent to include if PreviewProviderV2AdvancedCluster env var is enabled.
NOTE: This change will include this flag in ALL API calls from ALL resources & not limited to only advanced_cluster_tpf. To get actual analytics around this variable, we may need to filter by resource & org IDs.

Current user agent:

User-Agent: terraform-provider-mongodbatlas/devel Terraform/1.9.3

Updated format if PreviewProviderV2AdvancedCluster is enabled:

User-Agent: terraform-provider-mongodbatlas/devel Terraform/1.9.3 IsAdvancedClusterPreview/true

Link to any related issue(s): CLOUDP-309433

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

[lantoli]

why not follow the current format? e.g.

User-Agent: terraform-provider-mongodbatlas/devel Terraform/1.9.3 IsPreviewV2AdvancedClusterEnabled/true

(also consider reducing IsPreviewV2AdvancedClusterEnabled to something shorter but still easy to know what it is)

[maastha]

keeping existing format & renamed to IsAdvancedClusterPreview

Also will also include value if false as:
User-Agent: terraform-provider-mongodbatlas/devel Terraform/1.9.3 IsAdvancedClusterPreview/false

[marcosuma]

Can we double check this is not going to break any existing script that will eg start consider Terraform/1.9.3 IsAdvancedClusterPreview/true as a whole new entry? chances are low but good to ask Victor I guess

[lantoli]

is it possible to have some unit and/or acc tests?

[maastha]

not really, since the user agent method uses some static variables & TF version, it's tricky to add unit tests for this

[maastha]

Can we double check this is not going to break any existing script that will eg start consider Terraform/1.9.3 IsAdvancedClusterPreview/true as a whole new entry? chances are low but good to ask Victor I guess

@marcosuma checked with Victor, added link to the slack thread in the ticket

[maastha]

As discussed, holding off on merging this until after 1.33 release

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