Implement template scan_interval directive

[bdraco]

Needs #40685

Proposed change

Implement template scan_interval directive from home-assistant/architecture#206

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[bdraco]

• template tests
• event tests
• docs

[bdraco]

def _refresh is getting a bit too long so I'd like to refactor it

for track_template_ in self._track_templates:
    rendered, update = self._render_template_if_ready(
        track_template_,
        event,
        now
    )

[bdraco]

Also needs to skip updates if there is an event and we have an interval listener for the template so we don't trigger on entities from other templates