wifi(interface): Add Iface objects

Create a child object under IWifiChip to represent each interface
within the chip. Each iface object has a |type| & |ifname| which should
help us uniquely identify them. This should help us expose methods that
are applicable only to a specific interface type.

While there,
Assign a unique id to every chip on the device.
Add IWifi.listChipIds() to retrieve the list of chip Id's avaiable on the
device. IWifi.getChip() will now use the provided Id to retrieve the
corresponding IWifiChip object(because HIDL language doesn't support
vec<HIDL objects>).

Bug: 31943042
Bug: 32003988
Test: Interface compiles (not implementation)
Change-Id: I723007566ca4220362c02d0f452753fee4e31fce