parse/reconstruct messages with cid image references

[tomholub]

// todo - this test case is a stop-gap. We need to implement rendering of such messages below,
//   then let test plain messages with images in them (referenced by cid) just like other types of messages below

Originally posted by @tomholub in #3256 (comment)

We actually already do this on Android before, so we should have code for it available, in https://github.com/FlowCrypt/flowcrypt-mobile-core.