Add workflow to auto-deploy an OTA update

[gminn]

Summary

To streamline dog-fooding for this fleet, OTA updates are now triggered
when a push is made to the main branch of this repo.

Test Plan

Triggered the workflow manually with:

gh workflow run ota-deploy.yml --ref gminn/add-auto-ota-deploy

which completed successfully. The payload and symbol file (build id 9003536c6c192116291d676edf9dcdc79aca5fed) got uploaded correctly.
Verified that the ota deploy was successful by verifying my device got the payload
and did a firmware update:

*** Booting nRF Connect SDK 2d2da43e7515 ***
[00:00:00.271,545] <inf> mflt: Reset Reason, RESETREAS=0x10000
[00:00:00.271,606] <inf> mflt: Reset Causes:
[00:00:00.271,636] <inf> mflt:  Software
[00:00:00.272,735] <inf> mflt: GNU Build ID: 9003536c6c192116291d676edf9dcdc79aca5fed
[00:00:00.273,101] <inf> mflt: Periodic background upload scheduled - duration=4611s period=10800s
...
uart:~$ mflt get_device_info
[00:08:20.264,801] <inf> mflt: S/N: 351901930733237
[00:08:20.264,892] <inf> mflt: SW type: nrf91ns-fw
[00:08:20.264,953] <inf> mflt: SW version: 2024.01.08.7450362778
[00:08:20.264,984] <inf> mflt: HW version: thingy91

---

Resolves: MFLT-12995

[noahp]

Awesome!