feat: allow specifying custom relay in dumbpipe (#60)

[alx0x10]

Hello I'm back!

I thought this issue would be a good first step to get familiar with Iroh, so I started implementing the possibility to define a custom relay.
As mentioned in the issue, Sendme already supports this, so I mostly adapted that code.

Before going further and adding tests, I’d like your feedback on a few points:

• Do you think this feature makes sense here? Since Sendme already demonstrates it, maybe you prefer to keep dumbpipe shorter and focused only on the listen/connect logic?
• I introduced the anyhow dependency like in Sendme. Would you prefer a different error-handling approach? I could just implement the From<> myself.
• Like in Sendme, I wrapped RelayMode into RelayModeOption, which removes the Staging option. I haven’t really looked into how staging works, would you like me to explore that for this implementation?