export API to set RefTime of Updater #1039

Job	Run time
examples / multirepo (ubuntu-latest)	8s
examples / client (ubuntu-latest)	5s
examples / multirepo (macos-latest)	15s
examples / client (macos-latest)	16s
examples / multirepo (windows-latest)	1m 35s
examples / client (windows-latest)	1m 27s
examples / repository (ubuntu-latest)	13s
examples / root-signing (ubuntu-latest)	10s
examples / repository (macos-latest)	15s
examples / root-signing (macos-latest)	17s
examples / repository (windows-latest)	2m 21s
examples / tuf-client-cli (ubuntu-latest)	10s
examples / root-signing (windows-latest)	1m 23s
examples / tuf-client-cli (macos-latest)	16s
examples / tuf-client-cli (windows-latest)	1m 34s
linting / golangci-lint	2m 29s
linting / govulncheck	24s
tests / Run (ubuntu-latest)	32s
tests / Run (macos-latest)	43s
tests / Run (windows-latest)	1m 48s
	16m 21s

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