feat: device enumeration and sensor API

[abc8747]

Previously, opening a device by index was unreliable.

• added a new DeviceDescriptors struct that provides an iterator over only supported RTL-SDR devices, ensuring that DeviceId::Index(n) reliably refers to the nth known RTL-SDR
• a new Sensor API is introduced to provide an interface for querying device state like tuner type and gain
• the rtl_test example has also been updated to use the new enumeration logic, supporting device selection by either filtered index or serial number

Tested with blog V4:

$ cargo run --example rtl_test
Found 1 device(s):
  0:  RTLSDRBlog, Blog V4, SN: 00000001

Using device 0: RTLSDRBlog, Blog V4, SN: 00000001
Found r828d tuner
Supported gain values (29): 0.0 0.9 1.4 2.7 3.7 7.7 8.7 12.5 14.4 15.7 16.6 19.7 20.7 22.9 25.4 28.0 29.7 32.8 33.8 36.4 37.2 38.6 40.2 42.1 43.4 43.9 44.5 48.0 49.6
Sampling at 2048000 S/s.
Reading samples in sync mode...

Addresses xoolive/rs1090#264 (comment), xoolive/rs1090#294 (impl Send for Tuner and Error will be in a separate PR)
cc: @xoolive

[xoolive]

I just added some logic in the rtl_test example program to use a -f flag and select devices based on manufacturer/product/serial.
rust-soapysdr offers something similar and I find it very convenient as far as I am concerned.