Add MQTT homeassistant sensors for air quality

[glenrobertson]

Adds support for homeassistant sensor discovery and state reporting via MQTT

• Add NVS variable for MQTT url (e.g. mqtt://user:password@mqtt.example.com)
• Add MQTT lib which exposes a method for sending sensor data to a given MQTT url
• Add task to call MQTT method when there is an MQTT url present

Notes:

• I haven't yet tested the nvs variable setting - I tested with a hardcoded URL before adding this.
• The NVS code changes are based on the weather path NVS code.

Relevant home assistant docs for MQTT based sensor discovery:

• MQTT Discovery
• MQTT Sensor

Testing:

Blocking things to test:

• New web UI with old firmware (for people who haven't upgraded)
• New web UI with new firmware
• No change in behavior if MQTT server not set
• No change in behavior if MQTT server removed (Bug: Does not show MQTT url on connect)
• Stable for a few days with home assistant uploads

Non-blocking things to figure out:

• Any significant temperature difference if MQTT server set (wifi does heat up device)
• Packaging new release
• Web UI update to indicate firmware release

[babldev]

Thanks! Did a quick first pass just looking at code. I'll have to do a closer look running on device and testing it out. Specifically memory usage, and backwards compatibility with the web app.

[babldev]

Blocking things to test: (I can help with this too)

• New web UI with old firmware (for people who haven't upgraded)
• New web UI with new firmware
• No change in behavior if MQTT server not set
• No change in behavior if MQTT server removed (Bug: Does not show MQTT url on connect)
• Stable for a few days with home assistant uploads

Non-blocking things to figure out:

• Any significant temperature difference if MQTT server set (wifi does heat up device)
• Packaging new release
• Web UI update to indicate firmware release

[babldev]

Keeping some notes here. If you connect the new web UI to an old bitclock FW version, you will see:

• Failed to read characteristic: 2ab48c15-934d-11ec-b90a-802748c27b9e: NotFoundError: No Characteristics matching UUID 2ab48c15-934d-11ec-b90a-802748c27b9e found in Service with UUID ec2f3aa0-4ed8-71bd-f147-b2bc37195232.

If you try to set the MQTT url:
Uncaught (in promise) NotFoundError: No Characteristics matching UUID 2ab48c15-934d-11ec-b90a-802748c27b9e found in Service with UUID ec2f3aa0-4ed8-71bd-f147-b2bc37195232.

Other changes seem to work. So I think this is fine but would be nice to branch instead of error if it detects an old firmware version. But I'd need to reveal that somehow.

[babldev]

If you update to new FW, and connect with new web UI, it initially doesn't work at all

https://github.com/goat-hill/bitclock/tree/master/bitclock-fw#bluetooth-gatt-attributes

Once you "Forget this device" and re-pair there are no errors. But something to think about communicating if people update.

[babldev]

Annoyingly I think we're running out of memory when parsing the weather API now with cJSON. I bet we were close to the limit and this MQTT stuff pushed us over :(

I (7958) weather_api: HTTP GET Status = 200, content_length = 12906
E (7968) weather_api: Error on JSON parse

I'll have to poke around for ways to reduce memory usage.

[babldev]

Had to throw a ton of changes in here, mostly because the weather API is extremely expensive to the heap right now. It uses about 30kB via the cJSON library to parse the 17kB giant response from api.weather.gov.

I made a bunch of other tweaks in my testing that leaves about 3kB worst case of extra headroom. Not great because a small weather.api.gov change could push things over the limit.

Long term fix would be to use a streaming parser of JSON instead of a one-shot one. Those are harder to implement but way less memory intensive. https://components.espressif.com/components/espressif/jsmn instead of cJSON

[babldev]

I merged in master which should hopefully give this PR enough memory headroom since it was improved there.