[pos] Add epoch argument to vp::pos::rewards #1745

Job	Run time
build-binaries	8m 14s
changelog	17s
build-wasm (wasm, build-wasm-scripts, wasm/tx_.wasm wasm/vp_.wasm wasm/checksums.json )	4m 13s
check-packages	5m 59s
build-wasm (wasm-for-tests, build-wasm-tests-scripts, wasm_for_tests/*.wasm)	3m 48s
lints	5m 52s
rust-docs	6m 31s
test-ledger-app	6m 57s
test-unit	4m 6s
test-integration	25s
check-benchmarks	2m 3s
test-wasm	2m 2s
test-e2e-with-device-automation	1s
test-e2e	1s
upload-coverage	1s
	50m 30s

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