Add iam_data_access_service to appConfig

What does this MR do?

Introduces an optional IAM Data Access Service integration under global.appConfig.iamDataAccessService, mirroring the existing iam_auth_service but limited to a gRPC endpoint plus its own shared-secret token.

Consistent with iam_auth_service, the integration is gated behind an explicit enabled flag (default false):

• Self-managed (enabled: false, default): the iam_data_access_service block renders with enabled: false and the secret_file path, but no token is generated, nothing is mounted, and the configuration checks stay silent — so self-managed installs are never required to configure it.
• GitLab.com (enabled: true via its own deployment values): the chart renders the block with grpc.host/grpc.port into the webservice, sidekiq, and toolbox gitlab.yml, generates a dedicated authentication token, and mounts it on those deployments.

checkConfig requires grpc.host and grpc.port when the service is enabled (the checks only fire on enabled: true).

Related issues

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• Documentation created/updated.
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• Equivalent MR/issue for omnibus-gitlab opened.

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• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
• Consider downstream impact to the Operator, as per evaluating impact from changes to GitLab chart.