provider: add provider-level default_labels and apply them across label-capable resources

[Copilot]

Description

Adds provider-level default_labels so a workspace can define shared labels once and have them applied to resources that support labels. Resource-local labels still take precedence on key conflicts.

• Provider schema + config plumbing

  • Added default_labels to both provider implementations (exoscale SDKv2 and framework provider).
  • Propagated defaults through provider config objects for downstream resource access.

• Label merge/strip behavior

  • Added reusable helpers to:
    • convert Terraform maps to map[string]string
    • merge provider defaults with resource labels (resource values win)
    • strip defaults from state reads to avoid persistent diffs when labels are inherited only from provider config.

• Applied to label-capable resources

  • Wired merge-on-create/update and strip-on-read in resources currently handling labels across SDKv2/framework code paths (including compute, instance pool, SKS, NLB/private network/elastic IP, block storage, IAM role).

• Docs + focused tests

  • Updated provider docs/template to expose default_labels.
  • Added focused tests for provider schema exposure and label helper behavior.

provider "exoscale" {
  key    = var.exoscale_api_key
  secret = var.exoscale_api_secret

  default_labels = {
    tf_repo      = "https://example.org/my-iac-repo/paf"
    tf_workspace = terraform.workspace
  }
}

resource "exoscale_nlb" "example" {
  # ...
  labels = {
    tf_workspace = "override-for-this-resource"
  }
}

Checklist

(For exoscale contributors)

• Changelog updated (under Unreleased block)
• Acceptance tests OK
• For a new resource, datasource or new attributes: acceptance test added/updated

Testing

Targeted unit/package test coverage was added for schema/helper behavior; full acceptance coverage is left to CI/maintainer validation.