[Example] end-to-end example using a2a-go with external transport plugin

Hi All,

I'v developed a external transport plugin using a2a-go and wondering whether it belongs in `documentation/examples`

See the show and tell on A2A issue [https://github.com/a2aproject/A2A/discussions/2187](https://github.com/a2aproject/A2A/discussions/2187), the transport plugin for fmsg using a2a-go is here: [https://github.com/markmnl/a2a-go-fmsg](https://github.com/markmnl/a2a-go-fmsg)