Fix: DO-3224 EventBus events not firing for actions updating variables

Motivation and Context

There are cases where a Dara variable is not directly used by a useVariable hook, it can still 'live' in client-side state and be updated by actions. E.g.

from dara.core import Variable
from dara.components import Input

sync_to = Variable()

Input(onchange=sync_to.sync())

In systems relying on the event bus events to track the current state of the variable, in the scenario above there was never an event fired about the variable state changing since only useVariable used to fire events.

This PR adds explicit events about updated variables to the variable-mutating actions.

Implementation Description

Added event bus to the context injected into actions, grabbing from nearest capturer with useEventBus. Actions then simply publish events when they mutate variables

Any new dependencies Introduced

How Has This Been Tested?

Unit tests added Tested code in internal product

PR Checklist:

[x] I have implemented all requirements? (see JIRA, project documentation).
[x] I am not affecting someone else's work, If I am, they are included as a reviewer.
[x] I have added relevant tests (unit, integration or regression).
[ ] I have added comments to all the bits that are hard to follow.
[ ] I have added/updated Documentation.
[ ] I have updated the appropriate changelog with a line for my changes.

causalens / dara