Add AccessMode to Attribute

[shihab-dls]

Currently, we need to use this free function:

AccessModeType = Literal["r", "w", "rw", "d", "x"]


def _attribute_to_access(attribute: Attribute) -> AccessModeType:
    match attribute:
        case AttrRW():
            return "rw"
        case AttrR():
            return "r"
        case AttrW():
            return "w"
        case _:
            raise ValueError(f"Unknown attribute type {type(attribute)}")

to set the correct access mode in our PVA structure. It would be nicer if attributes had an access_mode property for this instead.

Acceptance Criteria

• Attributes have an access_mode property, and uses of the _attribute_to_access free function are replaced.