Mark the fine-grained apis as internal only

[anekkanti]

Note

Medium Risk
Changes API visibility metadata for the Custom Roles endpoints, which may remove them from public client generation/OpenAPI docs and impact any external consumers relying on them.

Overview
Marks the Custom Roles RPCs (GetCustomRoles, GetCustomRole, CreateCustomRole, UpdateCustomRole, DeleteCustomRole) as internal-only by adding google.api.method_visibility restrictions and importing google/api/visibility.proto.

Updates the OpenAPI tag description for Custom Roles to explicitly note the feature is Pre-Release and requires contacting Temporal Cloud support.

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