Add Experimental Endorsement Signalling #15711

Job	Run time
RPC and mobile compilation check	2m 47s
cross compilation	12m 41s
check commits	7m 54s
check pinned dependencies (google.golang.org/grpc v1.59.0)	4s
lint code	3m 50s
check pinned dependencies (github.com/golang/protobuf v1.5.3)	6s
sample configuration check	52s
Sqlc check	2m 38s
run unit tests (btcd unit-cover)	17m 13s
run macOS itest	48m 47s
run unit tests (unit tags="kvdb_etcd")	18m 55s
check release notes updated	4s
run unit tests (unit tags="kvdb_postgres")	18m 10s
run unit tests (unit tags="kvdb_sqlite")	16m 50s
run ubuntu itests (btcd, backend=btcd cover=1)	31m 18s
run unit tests (btcd unit-race)	36m 56s
run windows itest	38m 42s
run ubuntu itests (bitcoind, backend=bitcoind cover=1)	40m 51s
run unit tests (unit-module)	1m 11s
run ubuntu itests (bitcoind-notxindex, backend="bitcoind notxindex")	40m 13s
run ubuntu itests (bitcoind-rpcpolling, backend="bitcoind rpcpolling" cover=1)	41m 48s
run ubuntu itests (bitcoind-etcd, backend=bitcoind dbbackend=etcd)	54m 59s
run ubuntu itests (bitcoind-postgres, backend=bitcoind dbbackend=postgres)	1h 10m 21s
run ubuntu itests (bitcoind-sqlite, backend=bitcoind dbbackend=sqlite)	44m 7s
run ubuntu itests (bitcoind-postgres-nativesql, backend=bitcoind dbbackend=postgres nativesql=true)	1h 11m 55s
run ubuntu itests (bitcoind-sqlite-nativesql, backend=bitcoind dbbackend=sqlite nativesql=true)	44m 4s
run ubuntu itests (neutrino, backend=neutrino cover=1)	38m 40s
finish	3s
	11h 45m 59s

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