Add `only_on_change` flag to `add_on_update_callback`

[shihab-dls]

Allowing add_on_update_callback to take an only_on_change=True flag would allow us to fan out values to attributes bidirectionally. For example:

attr1 = AttrRW(Bool)
attr2 = AttrRW(Bool)

@dataclass
class AttrGroup:
    fan_out_attr: AttrRW
    
    async def __call__(self, value: Any):
        await fan_out_attr.update(value)
 
attr1.add_on_update_callaback(AttrGroup(attr2))
attr2.add_on_update_callaback(AttrGroup(attr1))

would re-trigger the callback recursively; but this:

attr1.add_on_update_callaback(AttrGroup(attr2), only_on_change=True)
attr2.add_on_update_callaback(AttrGroup(attr1), only_on_change=True)

would not. A use case for this can be found on #133.

Acceptance Criteria

• only_on_change flag implemented on add_on_update_callback
• Test written against new feature
• Tested on fastcs-PandaBlocks capture bits, closing Allow setting an AttrR/AttrRW without running the callbacks added downstream #133

[GDYendell]

Question for discussion: should we default to not calling callbacks if the value hasn't changed (and then maybe the flag is always=False)? This would be a change in behaviour.

@coretl?