Is your feature request related to a problem? Please describe.

Currently, when running sietch discover, all vaults on the same network exchange keys automatically. While this works fine for small home networks, it becomes unfeasible in larger environments such as an office, where users may not want to exchange keys with everyone.

Describe the solution you'd like
Allow users to selectively choose which peers they want to exchange keys with. The exchange should only occur when both parties have mutually selected each other. There should also be an option to select multiple peers at once.

Describe alternatives you've considered
Currently, the only option is to exchange keys with all peers automatically. No partial or selective exchange is available.

Additional context

graph LR
    style U1 fill:#f9f,stroke:#333,stroke-width:2px
    style U2 fill:#9f9,stroke:#333,stroke-width:2px
    style U3 fill:#9ff,stroke:#333,stroke-width:2px
    style U4 fill:#ff9,stroke:#333,stroke-width:2px

    U1((User 1))
    U2((User 2))
    U3((User 3))
    U4((User 4))

    %% Arrows indicate key exchange (mutual selection)
    U1 --> U2
    U2 --> U1
    U1 --> U3
    U3 --> U1
    U3 --> U4
    U4 --> U3

How it works:

1. Each node represents a user.
2. Arrows indicate mutual key exchange between users.
3. Only users with mutual selection have arrows in both directions.