build: release v5.15.0 - webhook attachments

[mantariksh]

This release deploys the changes from #1713 in isolation. These changes were reverted in #2169 in order to minimise risk for release v5.14.0, and are now being redeployed.

[mantariksh]

Manual tests

• Create a webhook receiving server with SDK version 0.9.0 which decrypts and prints the contents of attachments. Configure it to return 400 for the first request, and subsequently 200. Create a storage mode form with attachment fields, point the webhook URL to your receiving server and enable webhook retries. Check that the attachments are downloaded and decrypted correctly in both webhook attempts (the initial attempt which returns 400 and the first retry which returns 200).
• Configure the server to only decrypt the content, ignoring the attachments. Check that decryption still works.
• Remove the attachment field. Check that decryption still works when calling decrypt.
• Remove the attachment field. Check that decryption still works when calling decryptWithAttachments.
• Install SDK version <0.9.0 on the server and check that decryption works even when attachments are sent.
• Install SDK version <0.9.0 on the server and check that decryption works when attachments are not sent.