[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)

Review the linked discussion and the example plugin repository to understand the a2a-go SDK and the fmsg transport plugin. The task is to create an end-to-end example in the `documentation/examples` directory. Start by examining existing examples in the a2a-go repo, then integrate the plugin usage. Ensure the example runs and demonstrates the plugin's interaction with the A2A protocol.

Scritto dal modello di indicizzazione a partire dal testo della issue.