ESP32S2: Allow connecting to specific bssid

[astrobokonon]

Closes #3403

I couldn't figure out a way to avoid doing a last minute conditional check in common-hal/wifi/Radio.c because the bssid_set flag needs to be set only if you're specifying a specific MAC. If you're not passing in a bssid, but you happen to still set the bssid_set flag, the connection will fail and you'll end up returning WIFI_RADIO_ERROR_NO_AP_FOUND so that's no good (and a bit confusing).

I needed a way to check the bssid length as an initial sanity check against someone passing it in as a string (with colons) rather than the actual bytes, so I just dropped in #define MAC_ADDRESS_LEN when/where needed. I could have just hardcoded it but I saw that done elsewhere so I just did the same. It raises a ValueError but I wasn't sure if there was a better/more preferred type.

I don't see a way to get the vital statistics of the currently-connected AP, so that's the final thing to look at before I'd call this good to go. But it compiled and the initial tests worked as expected, which was frankly a surprise.

[tannewt]

One small potential bug. Otherwise this looks really good! What is left to do?

[astrobokonon]

@tannewt the only thing I'd like to do is to look at the set of parameters exposed after a connection is established to check/verify the details of the connection and know that you're connected where you think you are - right now there's really just common_hal_wifi_radio_get_ipv4_address to get the assigned IPv4 address, from which you can infer the subnet, but it'd be nice to have the gateway and the explicit subnet since they're all from the same esp_netif_ip_info_t struct.

[tannewt]

@astrobokonon What do you think about doing that in a follow up PR? This looks good as-is.

[astrobokonon]

@tannewt Yeah, I can do that in a different PR for sure.

[tannewt]

Looks great! Thank you! I'll let @dhalbert merge.