Add golioth_rpc feature

[ncmiller]

The RPC feature is not yet deployed on the backend (I tested with local golioth instance on branch feature/device-rpc). Therefore, the RPC feature is disabled by default in Kconfig.

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[ncmiller]

Note: We are failing checkpatch in CI right now due to CamelCase warnings related to usage of QCBOR, for example:

2a7e49ebff6c2a6e83543e2dc746944fda25f7a7:298: CHECK:CAMELCASE: Avoid CamelCase: <UsefulBufC>
#298: FILE: net/golioth/rpc.c:100:
+   UsefulBufC payload_buf = { payload, payload_len };

There's not much we can do about this since QCBOR is an external project, and checkpatch.pl is not smart enough to allow exceptions for that.

Therefore, we will ignore the checkpatch failure in this PR and invoke "GitHub superpowers" to force-merge after the review phase is complete.

[mniestroj]

There are build failures, which got unnoticed by CI: https://github.com/golioth/golioth-zephyr-sdk/runs/7396608549?check_suite_focus=true

I've observed this problem before and I think this was related to twister problem in Zephyr 3.1.0, which always returns 0 exit code even in case of errors. Zephyr from tip of main already fixed this issue, if I remember correctly.

[ncmiller]

There are build failures, which got unnoticed by CI: https://github.com/golioth/golioth-zephyr-sdk/runs/7396608549?check_suite_focus=true

Build error should be fixed now. I added #if defined(CONFIG_GOLIOTH_RPC) guards:

https://github.com/golioth/golioth-zephyr-sdk/pull/230/files#diff-47293fdec0466d8bad2826094caeca848de80536d9e1f918bc365c63d84e80c3R94-R97