auth: move PSK handling to samples

[sam-golioth]

• move system settings into samples

• add PSK support to hardcoded_credentials module

• remove PSK handling in system_client

• update samples to use the same auth for consistency

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit d6c3234):

https://golioth-zephyr-sdk-doxygen-dev--pr406-psk-refactor-ilew35wg.web.app

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[sam-golioth]

LGTM, in terms of code changes.

Would be good to have @golioth/developer-relations approval (doesn't have to be here).

Confirmed with @ChrisGammell