[sgen-bridge] Move the bridge implementation behind a pointer table.

This change is in preparation to introduce two implementations of the bridge.

The current implementation is now know as old.
[sgen-bridge] New bridge implementation - currently is just a copy of the old one.

The bridge implementation can be picked by having bridge-implementation=old|new in
MONO_GC_PARAMS.

This commit just prepared the ground to land experimental improvements to the bridge
while retaining an working implementation by default.
[sgen-bridge] Add the new bridge to the testing matrix.