[sr-dev] [kamailio/kamailio] msrp: update m=message media to proxy messages (#2186)

[sergey-safarov]

Think I search way to transparently inject kamailio as MSRP proxy into SDP and MSRP messages.
Looks as this behavior will be not standard. But I expect this feature will allow inject kamailio as MSRP proxy when I not able to control caller UA.

**SDP update**
Could you looks [RFC4976 Section 11. Example SDP with Multiple Hops](https://tools.ietf.org/html/rfc4976#section-11)

Here exist example
```
    c=IN IP4 bob.example.com
    m=message 1234 TCP/TLS/MSRP *
    a=accept-types: message/cpim text/plain
    a=path:msrps://relay.example.com:9000/hjdhfha;tcp  \
           msrps://bob.example.com:1234/fuige;tcp
```
Attribute `a=path` have two `msrps` uri.

I expect add kamailio msrp socket into `a=path` attribute.
Like Kamailio receive caller SDP like
```
    c=IN IP4 bob.example.com
    m=message 1234 TCP/TLS/MSRP *
    a=accept-types: message/cpim text/plain
    a=path:msrps://bob.example.com:1234/fuige;tcp
```
Then add `msrps://relay.example.com:9000/hjdhfha;tcp  \` at top of `a=path` and forwards to FreeSwitch (Asterisk) daemon.

And then make similar SDP update for `183` and '200' responses.

**MSRP messages updates**
Caller send
```
    MSRP 6aef SEND
    To-Path: msrps://b.example.net:9000/aeiug;tcp \
             msrps://bob.example.net:8145/foo;tcp
    From-Path: msrps://alice.example.org:7965/bar;tcp
    Success-Report: yes
    Byte-Range: 1-*/*
    Message-ID: 87652
    Content-Type: text/plain

    Hi Bob, I'm about to send you file.mpeg
    -------6aef$
```
Kamailio forward
```
    MSRP 6aef SEND
    To-Path: msrps://bob.example.net:8145/foo;tcp
    From-Path: msrps://b.example.net:9000/aeiug;tcp \
             msrps://alice.example.org:7965/bar;tcp
    Success-Report: yes
    Byte-Range: 1-*/*
    Message-ID: 87652
    Content-Type: text/plain

    Hi Bob, I'm about to send you file.mpeg
    -------6aef$
```

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