Allow egress notification subscriptions from external accounts

[manics]

Currently the SNS topics used to send notifications about new egress requests assumes the subscriber is in the same account:

TREEHOOSE/src/components/egress_app_backend/egress_backend/egress_backend_stack.py

Lines 669 to 709 in 7fc18fe

	# Publish notifications to SNS
	notify_ig_reviewer_task = sfn_tasks.SnsPublish(
	self,
	"Notify Information Governance",
	topic=ig_role_topic,
	subject=sfn.JsonPath.string_at(
	"States.Format('{} Egress Request', $.project_id)"
	),
	message=sfn.TaskInput.from_object(
	{
	"Egress Request ID": sfn.JsonPath.string_at("$.egress_request_id"),
	"Researcher Email": sfn.JsonPath.string_at("$.created_by_email"),
	"Egress Object File Types": sfn.JsonPath.string_at(
	"$.copy_to_staging_result.file_extensions"
	),
	}
	),
	result_path=sfn.JsonPath.DISCARD,
	)
	notify_rit_reviewer_task = sfn_tasks.SnsPublish(
	self,
	"Notify Research IT",
	topic=rit_role_topic,
	subject=sfn.JsonPath.string_at(
	"States.Format('{} Egress Request', $.project_id)"
	),
	message=sfn.TaskInput.from_object(
	{
	"Egress Request ID": sfn.JsonPath.string_at("$.egress_request_id"),
	"Researcher Email": sfn.JsonPath.string_at("$.created_by_email"),
	"Information Governance": sfn.JsonPath.string_at(
	"$.information_governance.result.ig_reviewer_1_email"
	),
	"Egress Object File Types": sfn.JsonPath.string_at(
	"$.copy_to_staging_result.file_extensions"
	),
	}
	),
	result_path=sfn.JsonPath.DISCARD,
	)

We could add a parameter that allows a central subscriber (e.g. a Lambda running in a central TRE account that creates Microsoft Teams notifications or JIRA tickets) to subscribe using AddToResourcePolicyResult