refactor(zetaclient): extract base observer and base signer from BTC #3587

Job	Run time
build-zetanode	3m 0s
matrix-conditionals	4s
start-e2e-consensus-test / e2e	0s
start-e2e-performance-test / e2e	0s
start-e2e-import-mainnet-test / e2e	0s
start-e2e-admin-test / e2e	0s
start-legacy-test / e2e	0s
start-e2e-test / e2e	9m 20s
start-solana-test / e2e	0s
start-ton-test / e2e	0s
start-tss-migration-test / e2e	0s
start-upgrade-import-mainnet-test / e2e	0s
start-upgrade-test / e2e	0s
start-upgrade-test-light / e2e	0s
e2e-ok	2s
	12m 26s

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