librpc/idl: Add idl for tpm20_rsakey_blob

librpc/idl: Add idl for tpm20_rsakey_blob

Idl and tests for TPM20_RSAKEY_BLOB, one of the possible encoding of
msDSKeyCredentialLink KeyMaterial
Derived from:
    https://dox.ipxe.org/Tpm20_8h_source.html#l00164
    https://stackoverflow.com/questions/78958315/cannot-parse-tpm2-0-public-key

Note: this is a greatly simplified implementation that only handles TPM
      version 2, RSA public keys.

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>