Enable discovery to continue if reading a holding register hangs

[tazle]

Hi,

We are dealing with a device that claims to be Sunpec-compliant, but fails to respond to reads of holding register 0, making it impossible to use the standard discovery process as implemented in the sunspec module.

Some options to resolve the issue:

1. Time out reads, so the process could proceed from 0 onwards
2. Enable specifying the starting address for discovery

[bikeshedder]

I think I'll add a discovery config struct for those two things:

struct Discovery {
    /// Addresses to check for the SunS identifier (default: [0, 40000, 50000])
    info_model_addrs: Vec<u16>,
    /// Timeout to be applied for every `read_holding_register` call
    read_timeout: Duration,
}

[tazle]

This sounds excellent.