Allow reuse of the same sender email across multiple workspaces/products

Description: Enable a single verified sender email/address to be shared across multiple products/workspaces within the same organization. Org admins can verify an email/domain once and grant access to that sender identity to selected workspaces, so each can send with the same "From" address. Problem: The product currently blocks reusing the same sending address across workspaces, forcing different addresses per workspace. For organizations running separate workspaces (e.g., by language) under one brand, this causes inconsistent "From" identities, recipient confusion, extra mailbox management, and fragmented deliverability/compliance setups. Benefit: Consistent brand identity across all communications Simpler DNS/verification management (SPF/DKIM/DMARC) with one setup Reduced operational overhead (fewer mailboxes and verifications) Clearer analytics and deliverability control per workspace while preserving a unified sender Better recipient trust and lower risk of phishing confusion