Use `crypto/ecdh` for tpmdirect

It seems like `tpmdirect` is dealing with a bit of low-level stuff to accomplish ECDH which is mostly solved with the new `crypto/ecdh` package.

Would it be a good idea to consider using this package maybe?

Currently the only missing part is `TPMECCNistP224` which `crypot/ecdh` doesn't implement.

Start by locating tpmdirect's low-level ECDH implementation and comparing it with Go's crypto/ecdh package. Check how the missing TPMECCNistP224 case affects the proposed change; done means tpmdirect uses the standard package where supported without losing required TPM curve support.

Written by the indexing model from the issue text.