feat: add support for latest downstream cognito-message-sender

[sgtoj]

• add allow list to by email verifications
• use sendgrid an email provider

[Copilot]

Pull Request Overview

This PR adds support for the latest downstream cognito-message-sender by introducing email provider configuration and SendGrid-specific enhancements. The changes enable multi-provider email support with validation rules and add email verification allowlist functionality.

• Introduces configurable email providers with validation for exactly one provider selection
• Splits SendGrid API keys into separate send and verification keys with backward compatibility
• Adds email domain allowlist for bypassing email validation

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
variables.tf	Adds email provider configuration, new SendGrid API key variables, and validation rules
main.tf	Updates Lambda environment variables to use new provider configuration and SendGrid keys

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