Add method to mark message as read to REST API

When exposing unread messages counters in
https://code.briarproject.org/briar/briar/-/merge_requests/1283, I
noticed that they were never set to 0.

Fixes #1780

briar-headless/README.md

@@ -234,6 +234,19 @@ The text of the message should be posted as JSON:
 }
 ```
 
+### Marking private messages as read
+
+`POST /v1/messages/{contactId}/read`
+
+The `messageId` of the message to be marked as read
+needs to be provided in the request body as follows:
+
+```json
+{
+    "messageId": "+AIMMgOCPFF8HDEhiEHYjbfKrg7v0G94inKxjvjYzA8="
+}
+```
+
 ### Listing blog posts
 
 `GET /v1/blogs/posts`