Add always flag to add_on_update_callback

[shihab-dls]

closes #306 and closes #133

Adds an always flag to add_on_update_callback, and checks this flag when deciding what callback to call in update.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.69%. Comparing base (c43f71b) to head (470d6b2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #311   +/-   ##
=======================================
  Coverage   90.69%   90.69%           
=======================================
  Files          70       70           
  Lines        2535     2537    +2     
=======================================
+ Hits         2299     2301    +2     
  Misses        236      236           

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[shihab-dls]

Tested with fastcs-PandaBlocks; BitGroupOnUpdate can be reduced to:

@dataclass
class BitGroupOnUpdate:
    """Bits are tied together in bit groups so that when one is set for capture,
    they all are.

    This callback sets all capture attributes in the group when one of them is set.
    """

    capture_attribute: AttrRW[enum.Enum]
    bit_attributes: list[AttrRW[bool]]

    async def __call__(self, value: Any):
        if isinstance(value, enum.Enum):
            bool_value = bool(self.capture_attribute.datatype.index_of(value))  # type: ignore
            enum_value = value
        else:
            bool_value = value
            assert isinstance(self.capture_attribute.datatype, Enum)
            enum_value = self.capture_attribute.datatype.members[int(value)]

        await asyncio.gather(
            *(bit_attr.update(bool_value) for bit_attr in self.bit_attributes),
            self.capture_attribute.update(enum_value),
        )

removing the need for:

async def _set_attr_if_not_already_value(attribute: AttrRW[DType_T], value: DType_T):
    if attribute.get() != value:
        await attribute.update(value)

So, this PR will be updated to close #133, as well.