use chat protocol and contacts in chat commands/messages (#66)

* chat types, chat protocol syntax idea

* chat message syntax, raw message type

* chat message format and parsing

* raw chat message parsing test

* add message parsing tests

* interpret RawChatMessage

* use chat message format when sending messages

* save contacts and related connections to DB (WIP)

* use contacts in all chat commands (add, connect, send, delete)

* use contacts when receiving messages and notifications

* handle contact not found error

* automatically accept connection when CONF is received from the agent

src/Simplex/Chat/protocol.md

@@ -0,0 +1,70 @@
+# Chat protocol
+
+## Design constraints
+
+- the transport message has a fixed size (8 or 16kb)
+- the chat message can have multiple parts/attachments
+- the chat message including attachments can be of any size
+- if the message is partially received, it should be possible to parse and display the received parts
+
+## Questions
+
+- should content types be:
+  - limited to MIME-types
+  - separate content types vocabulary
+  - both MIME types and extensions
+  - allow additional content types namespaces
+
+## Message syntax
+
+The syntax of the message inside agent MSG:
+
+```abnf
+agentMessageBody = message / msgContinuation
+message = [chatMsgId] SP msgEvent SP [parameters] SP [contentParts [SP msgBodyParts]]
+chatMsgId = 1*DIGIT ; used to refer to previous message;
+                    ; in the group should only be used in messages sent to all members,
+                    ; which is the main reason not to use external agent ID -
+                    ; some messages are sent only to one member
+msgEvent = protocolNamespace 1*("." msgTypeName)
+protocolNamespace = 1*ALPHA ; "x" for all events defined in the protocol
+msgTypeName = 1*ALPHA
+parameters = parameter *("," parameter)
+parameter = 1*(%x21-2B / %x2D-7E) ; exclude control characters, space, comma (%x2C)
+contentParts = contentPart *("," contentPart)
+contentPart = contentTypeNamespace "." contentType ":" contentSize [":" contentHash]
+contentType = "i." <mime-type> / contentTypeNamespace "." 1*("." contentTypeName)
+contentTypeNamespace = 1*ALPHA
+contentTypeName = 1*ALPHA
+contentHash = <base64>
+msgBodyParts = msgBodyPart *(SP msgBodyPart)
+msgEventParents = msgEventParent *msgEventParent ; binary body part for content type "x.dag"
+msgEventParent = memberId refMsgId refMsgHash
+memberId = 8*8(OCTET) ; shared member ID
+refMsgId = 8*8(OCTET) ; sequential message number - external agent message ID
+refMsgHash = 16*16(OCTET) ; SHA256 of agent message body
+msgContinuation = "#" prevMsgId "." continuationId continuationData
+```
+
+### Example: messages, updates, groups
+
+```
+"3 x.msg.new c.text c.text:5 hello "
+"4 x.msg.new c.image i.image/jpg:256,i.image/png:4096 abcd abcd "
+"4 x.msg.new c.image x.dag:32,i.image/jpg:8000:hash1,i.image/png:16000:hash2 binary1"
+"#4.1 binary1end binary2"
+"#4.2 binary2continued"
+"#4.3 binary2end "
+"5 x.msg.new c.image i.image/jpg:256,i.image/url:160 abcd https://media.example.com/asdf#abcd "
+'6 x.msg.update 3 c.text:11,x.dag:16 hello there abcd '
+'7 x.msg.delete 3'
+'8 x.msg.new app/v1 i.text/html:NNN,i.text/css:NNN,c.js:NNN,c.json:NNN ... ... ... {...} '
+'9 x.msg.eval 8 c.json:NNN {...} '
+'10 x.msg.new c.text 2 c.text:16,x.dag:32 hello there @123 abcd '
+' x.grp.mem.inv 23456,123 1 c.json NNN {...} '
+' x.grp.mem.acpt 23456 1 c.text NNN <invitation> '
+' x.grp.mem.intro 23456,234 1 c.json NNN {...} '
+' x.grp.mem.inv 23456,234 1 c.text NNN <invitation> '
+' x.grp.mem.req 23456,123 1 c.json NNN {...} '
+' x.grp.mem.direct.inv 23456,234 1 text NNN <invitation>  '
+```