Implement atcab_write_pubkey and atcab_priv_write commands

`atcab_write_pubkey`: Uses the write command to write a public key to a slot in the proper format.
* https://microchiptech.github.io/cryptoauthlib/a06481.html#ga47cbedeb3c3e9f7cbf2b9433cd60d1aa

`atcab_priv_write`: Executes PrivWrite command, to write externally generated ECC private keys into the device.
* https://microchiptech.github.io/cryptoauthlib/a06481.html#gad01e8e785073d8655de9116e7df8e7b5

uCryptoAuthLib Implementation for `pubkey` (`priv_write` is not implemented): https://github.com/dmazzella/ucryptoauthlib/blob/master/cryptoauthlib/basic.py#L852

Discussion about these commands: https://github.com/MicrochipTech/cryptoauthlib/issues/44

Read the CryptoAuthLib documentation for atcab_write_pubkey and atcab_priv_write, then compare the referenced uCryptoAuthLib implementation in cryptoauthlib/basic.py around line 852. Trace how this repository exposes device commands and verify that both public-key and externally generated private-key writes use the documented formats and behavior.

Written by the indexing model from the issue text.