Research solution for providing iframe URL to offscreen execution environment

Currently we wrap each stream message with the iframe URL to the offscreen execution environment:

https://github.com/MetaMask/snaps-monorepo/blob/c7cf8b19a6c2570377f4f5163bfb0a035941c145/packages/snaps-controllers/src/services/offscreen/OffscreenPostMessageStream.ts#L65-L74

Ideally we have a better solution, allowing the extension to configure the iframe URL, without adding it to each stream message.

Start with packages/snaps-controllers/src/services/offscreen/OffscreenPostMessageStream.ts at the linked lines where each stream message is wrapped with the iframe URL. Investigate how the extension could configure that URL separately from stream messages. Done means documenting and validating a concrete approach that removes the per-message URL requirement.

Written by the indexing model from the issue text.