Add user-defined certificate and private key handlers to ESP32SPI

[brentru]

Adding set_certificate and set_private_key to ESP32SPI to reflect adafruit/nina-fw#11

Simpletest (note, version is 1.4.0 reflecting nina-fw pr #11):

code.py output:
ESP32 SPI webclient test
ESP32 found and in idle mode
Firmware vers. bytearray(b'1.4.0\x00')
MAC addr: ['0x28', '0x7f', '0x11', '0x12', '0xcf', '0xa4']
Connected to NETWORK 	RSSI: -44
My IP address is 
IP lookup adafruit.com: 104.20.38.240
Ping google.com: 10 ms
Fetching text from http://wifitest.adafruit.com/testwifi/index.html
----------------------------------------
This is a test of Adafruit WiFi!
If you can read this, its working :)

----------------------------------------

Fetching json from http://api.coindesk.com/v1/bpi/currentprice/USD.json
----------------------------------------
{'time': {'updated': 'Oct 8, 2019 20:49:00 UTC', 'updatedISO': '2019-10-08T20:49:00+00:00', 'updateduk': 'Oct 8, 2019 at 21:49 BST'}, 'disclaimer': 'This data was produced from the CoinDesk Bitcoin Price Index (USD). Non-USD currency data converted using hourly conversion rate from openexchangerates.org', 'bpi': {'USD': {'code': 'USD', 'description': 'United States Dollar', 'rate_float': 8144.39, 'rate': '8,144.3933'}}}
----------------------------------------
Done!

Testing esp32spi_aio_post (uses WiFiManager)

code.py output:
ESP32 SPI webclient test
Posting data...{'lat': None, 'feed_id': 997503, 'ele': None, 'expiration': '2019-12-07T20:48:22Z', 'id': '0E931MSGWB4BV8GF0D5BCXV65P', 'lon': None, 'value': '0', 'feed_key': 'test', 'location': None, 'created_at': '2019-10-08T20:48:22Z', 'created_epoch': 1570567702}
OK

[ladyada]

looks good can merge on pass