send MessageID to FES for easier threading of conversations on Gmail

[tomholub]

Roman - on FES we allowed two methods to supply message ID (the mime email header).

Option 1: pre-generate message id before constructing mime / uploading the message to Gmail, and include that in FES /upload call

Option 2: let Gmail generate message id (as it does now) and then retrieve it & send that to FES in another API request

Option 1 is preferred if it can be made to work (if Gmail respects message id constructed by us when we're sending the message) as it requires fewer API calls.

Pre-generate option:

• create a valid Message-ID - these should include the angled brackets.
• Send it along in the MessageUploadRequest class:

data class MessageUploadRequest(
   val associateReplyToken: String,
   val expiration: Instant?,
   val from: String,
   val to: List<String>,
   val cc: List<String> = emptyList(),
   val bcc: List<String> = emptyList(),
   val emailGatewayMessageId: String? = null
)

Separate API request option:

• after sending the email retrieve the Message-ID.
• Send it to the new POST
  /api/$VERSION_CLIENT/message/{externalId}/gateway endpoint.
  Authorization is required and the class is:

data class MessageGatewayUpdateRequest(
   val emailGatewayMessageId: String,
)

[tomholub]

related https://github.com/FlowCrypt/flowcrypt-enterprise-on-prem/pull/2365

and https://www.jwz.org/doc/mid.html

Please test the option 1, and see if Gmail accepts it & it stays the same. If yes, we can use that option.

[rrrooommmaaa]

Unfortunately Gmail overwrites Message-ID and don't even keep the original value in X-Google-Original-Message-ID as it used to do, so we need to proceed to Option 2.
I am a bit baffled by this externalId in /api/$VERSION_CLIENT/message/{externalId}/gateway endpoint.
Looks like neither NewReplyToken nor messageUpload return it.

[tomholub]

Indeed, it's not returned separately. Please implement it as if externalId was returned from messageUpload in MessageUploadResponse alongside url. Then I'll have that change done on FES.