Make DebugClient easier to extend (eg. by making private things protected)

[DanTup]

The DebugClient class is excellent for testing DAs, but there are some things missing that are more effort to add than they could be.

For example, the hitBreakpoint method works great for launch requests but can't be used for attach requests (since it does the launch itself). Making our own copy of hitBreakpoint means also copying a lot of the private methods resulting in a lot of duplication.

It'd be great if everything was made protected so we could extend it.

(For the specific example above, maybe hitBreakpoint could also check the launch args and attach if it's an attach request?)