Only create shared secret, when hmac is requested for CTAP2.0 tokens #434

Job	Run time
build (ubuntu-latest, x86_64-unknown-linux-gnu, true, sudo apt-get update sudo apt-get install -y...	1m 25s
build (ubuntu-latest, x86_64-unknown-linux-gnu, true, sudo apt-get update sudo apt-get install -y...	2m 22s
build (ubuntu-latest, x86_64-unknown-linux-gnu, true, sudo apt-get update sudo apt-get install -y...	1m 29s
build (macos-latest, x86_64-apple-darwin, true, --features crypto_dummy --no-default-features --t...	1m 40s
build (windows-latest, x86_64-pc-windows-gnu, true, --features crypto_dummy --no-default-features...	2m 24s
build (ubuntu-latest, x86_64-unknown-freebsd, false, sudo apt-get update sudo apt-get install -y ...	1m 2s
build (ubuntu-latest, x86_64-unknown-netbsd, false, sudo apt-get update sudo apt-get install -y l...	57s
build (ubuntu-latest, x86_64-linux-android, false, sudo apt-get update sudo apt-get install -y li...	54s
	12m 13s

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