Export SubscriptionActivity for use when driving the session event-loop #371

Job	Run time
build-linux (stable)	3m 34s
build-windows (stable)	12m 19s
build-linux (beta)	3m 36s
build-windows (beta)	9m 55s
code-coverage / cov	2m 28s
verify-clean-address-space / address_space	10s
verify-clean-node-ids / node_ids	6s
verify-clean-status-codes / status_codes	9s
verify-clean-supported-message / supported_message	10s
verify-clean-types / types	9s
verify-code-formatting / fmt	13s
	32m 49s

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