Transport endpoint enumeration

[fredzo]

Hi Andrew,

This is the 2nd PR of a set of 3 designed to add transport endpoint enumeration as proposed bu @sugarthc on scopehal Discord channel (goes with ngscopeclient/xptools#38).
This scopehal part covers:

• SCPITransport::EnumTransportEndpoints() that list available endpoints for a given transport.
• Default implementation returns an empty list.
• SCPIHIDTransport implementation lists HID devices currently connected to the PC
• SCPIUARTTransport implementation lists available COM ports

SCPITMCTransport is not covered by this PR since I don't have the means to test it, but maybe @sugarthc will be willing to contribute his work.