Add vendor callback for receiving spinel frames

Adds a vendor callback mechanism to the ThreadChip class. This allows
vendors to implement product-specific features by registering a callback
function that will be invoked when ever a spinel frame is received.

- Added `mVendorCallback` member to store the vendor callback.
- Added `setVendorCallback` method to set the vendor callback.
- Modified `handleReceivedFrame` to invoke the vendor callback with the received spinel frame.

Bug: 371460661
Test: build and flash
Change-Id: I97b80db08cfcc2cade520c55ecc27a50a1727690