Added ECDH support

[CaptainHaddock2016]

Added support for Elliptic Curve Diffie-Hellman key agreement. This function is present on the ATECC608A/B as per the datasheet.

[FoamyGuy]

This looks okay to me. I tried to test it, but was unable to get the ATECC608 breakout working.

It seems every function I try with my breakout including the new ECDH function results in Execution Error status response from the ATECC. I get the same results with gen_key function as well. I am guessing that I have gotten my ATECC into some locked or bad state where it doesn't want to perform these actions. The datasheet isn't really clear about what this error indicates, and I don't think I want to chase it any further right now. I notice several other issues relating to "CRC errors" which look like similar if not the same symptoms I am seeing. The CRC check fails because the buffers are sized assuming successful output, the error codes are a much smaller output so they need to use a smaller buffer in order to calculate the proper CRC. I used debug=True to see the raw data and can tell from there that 0xf the constant for "Execution Error" is always being returned for every function I try.

I did confirm the ECDH op code used by this PR in the datasheet and the function for it appears to me like it should be working with an ATECC that is in an operable state.