Export SubscriptionActivity for use when driving the session event-loop #372

Job	Run time
build-windows (stable)	10m 28s
build-linux (stable)	3m 37s
build-windows (beta)	10m 30s
build-linux (beta)	3m 37s
code-coverage / cov	2m 16s
verify-clean-address-space / address_space	9s
verify-clean-node-ids / node_ids	8s
verify-clean-status-codes / status_codes	8s
verify-clean-types / types	7s
verify-clean-supported-message / supported_message	8s
verify-code-formatting / fmt	6s
	31m 14s

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