implement currentFPS for SRT

[ruirodrigues-eyenov]

Hello
I was looking in how to have currentFPS for updated SRTStream similar to how it is implemented in RTMPStream/RTMPConnection.
However the currentFPS in NetStream setter is internal so SRTStream cannot write into this property.

One option is to make it public, similar to readyStateWillChange(to:) , or use the new package access modifier available in swift 5.9.

[shogo4405]

Creating a method for currentFPS seems excessive. It might be a good idea to remove the definition of NetStream.currentFPS and define it in SRTStream and RTMPStream instead.