`Session` should have it's buffer passed in from outside

[GnomedDev]

Currently, this buffer leads to stack overflows in my project so I would like to allocate it statically. https://github.com/esp-rs/esp-mbedtls/blob/27c781fd7a4178e574072aee7ce8821c9ab07335/esp-mbedtls/src/lib.rs#L686-L687

[bjoernQ]

I agree - letting the user supply buffers is probably a good thing. However, since its generic over the BUFFER_SIZE the user can at least control the size of the buffer already