This PR implements support for AES using Galois Counter Mode, a mode for Authenticated Encryption with Additional Data (AEAD) that protects not only the confidentiality of the plaintext but the integrity of both the plaintext and optional additional data. AESGCM is widely used in internet protocols, is included included in many standards and is supported by the cryptography CPython library.

This implementation currently only supports systems that use mbedTLS for cryptography and not axTLS. This is due to axTLS both not including the necessary code and being effectively a dead project from a development point of view.

The new functionality is enabled by setting the MICROPY_PY_UCRYPTOLIB_GCM flag in the mpconfigport.h header. The PR sets this by default for the esp32 build (since it builds with mbedTLS already and the size increase is modest).

The PR also includes documentation for the new class and methods.

Note that the function signatures for the encrypt() and decrypt() methods differ from the ones for the basic aes class. This is because AEAD operations both require more inputs and have different semantics to the underlying block cipher operations. This is also why the functionality was implemented as a new class rather than a new mode on the existing class.