feat: upgrade to stellar protocol 27 (#549) #1596

Job	Run time
build	6m 27s
test reporter	2m 26s
test scaffold-cli (build_clients)	6m 43s
test scaffold-cli (examples 1-14)	7m 44s
test scaffold-cli (features)	6m 27s
test scaffold-cli (examples 15-27)	8m 0s
	37m 47s

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