rpc: add gettx command to walletrpc #12354

Job	Run time
Sqlc check	30s
check commits	1m 18s
cross compilation	26m 10s
lint code	4m 41s
check pinned dependencies (google.golang.org/grpc v1.56.3)	4s
RPC and mobile compilation check	1m 57s
sample configuration check	29s
check pinned dependencies (github.com/golang/protobuf v1.5.3)	3s
run unit tests (btcd unit-cover)	14m 11s
check release notes updated	5s
run unit tests (unit tags="kvdb_etcd")	16m 4s
run unit tests (unit tags="kvdb_postgres")	14m 34s
run ubuntu itests (btcd, backend=btcd)	44m 29s
run windows itest	52m 28s
run ubuntu itests (bitcoind, backend=bitcoind)	53m 41s
run unit tests (unit tags="kvdb_sqlite")	14m 1s
run ubuntu itests (bitcoind-notxindex, backend="bitcoind notxindex")	54m 57s
run unit tests (btcd unit-race)	32m 11s
run ubuntu itests (bitcoind-rpcpolling, backend="bitcoind rpcpolling")	54m 12s
run unit tests (unit-module)	50s
run ubuntu itests (bitcoind-etcd, backend=bitcoind dbbackend=etcd)	1h 9m 25s
run ubuntu itests (bitcoind-postgres, backend=bitcoind dbbackend=postgres)	1h 23m 56s
run ubuntu itests (bitcoind-sqlite, backend=bitcoind dbbackend=sqlite)	57m 52s
run ubuntu itests (neutrino, backend=neutrino)	52m 57s
	10h 51m 5s

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