feature requests: call hold, unhold and (blind/attended) transfer

[prinze77]

Implement new features

1-callHold max 3 calls
-Method that will put on hold the current call.

2-callUnhold
-Method that will connect back to the call that is on hold

3-blindTransfer
-Method that will transfer current call to other extension

4-attendedTransfer
-Method that will put active call on hold and will call to other extension then give you option transfer the call that is on hold or connect back to the call that is on hold
Sometimes you need to talk with the one who will get the transfer call before you transfer the call

Attaching a screenshot of UI that will use this features

[Mazuh]

k, I'll expose these APIs for you today

[jchavanton]

@prinze77 the look and feel of your UI is looking good, are you going to open source it ?

[prinze77]

@Mazuh I understand how hard is to implement this functionality. My brain feels so pain when im trying to build the logic around the active sessions storage.

Thank you for your interest @jchavanton and yeah, im planing to make it like a simple react component, but first we need to implement functionality to flowroute class.
https://github.com/prinze77/react-softphone

I will use jssip_client lib as soon as we get the requested methods.