Support importing HMAC keys and operations

FR to support
* importing external HMAC key
* hmac some data with it

to quote offline thread

go-tpm does not support `tpm2_hmac_start` yet, but it does support hash sequences (so the only command that needs to be added is tpm2_hmac_start).

With tpm2_hmac_start in hand, you could import a key and use it for hmacs through the `tpm2_hmac_start/tpm2_sequenceupdate/tpm2_sequencecomplete` flows. Implementing this should be pretty similar to this [PR](https://github.com/google/go-tpm/commit/289acaa6d003e6cb06492d3fdc9c4fcc4c292619).

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`tpm2_tools` currently does not support importing hmac but does support ::
- [tpm2_hmac](https://github.com/tpm2-software/tpm2-tools/blob/master/man/tpm2_hmac.1.md)