Integrate Nonin OEM III into firmware

[ethanjli]

This PR:

• Integrates SpO2 sensing into the main loop of the firmware
• Slightly renames some types in the Nonin OEM III driver
• Slightly refactors/adapts the driver into the uniform low-level Device and higher-level Sensor interface pattern which we have started using with the SFM3019 driver.
• Solves a new problem in Github Actions firmware checks (missing public key for installing LLVM and clang from LLVM's website rather than Github's custom distribution which is unable to install clang-tools)

[ethanjli]

This PR is in the critical path of development, passes clang-tidy and clang-format checks, is needed for me to make progress on the milestone 8 (v0.5), is of at least comparable quality to the rest of the firmware codebase, and probably would not be able to find someone to review it very quickly. We will need to go back and review everything in this PR and refactor and add another layer to the Nonin OEM III driver code, which may induce some refactoring of code from this PR. I will merge in this PR now.

For records-keeping:

1. This project is licensed under Apache License v2.0 for any software, and Solderpad Hardware License v2.1 for any hardware - do you agree that your contributions to this project will be under these licenses, too? Yes
2. Were any of these contributions also part of work you did for an employer or a client? No
3. Does this work include, or is it based on, any third-party work which you did not create? No