correct Radio.connect() and .start_ap() signatures; clean up some code

[dhalbert]

In the course of testing #6897 (could not reproduce), I found that the signatures for wifi.Radio.connect() and wifi.Radio.start_ap() did not correspond exactly to what these functions took, and that arg validation could use some improvement.

• Corrected type annotations for wifi.Radio.connect() and wifi.Radio.start_ap().
• Use b"" and None where appropriate in those methods, rather than MP_OBJ_NULL, which doesn't have a direct analog.
• Use bitmask values for wifi.AuthMode instead of shifting the integer values.
• Use uint32_t instead of uint8_t for passing around combinations of AuthMode, in case there are more than 8 bits in the future. It may be less code anyway.

Right now you give multiple AuthModes by putting them in a tuple. But they could be or'd (|) together instead now that they are or'able flags. Maybe in the future that should be the preferred way. We could also add combo AuthModes for the combinations Espressif supports, and what CYW43 supports.

I used b"" as the default empty string value. Internally it is mp_const_empty_bytes. Surprisingly there is no mp_const_empty_str.

I tested connect() with a local open access point. I tested start_ap() as an open and a password-protected AP. I was able to connect and ping it from a laptop in both cases.

[tannewt]

Looks good! Thank you!