samples: hello: add helper script for generating keys/certs

[mniestroj]

Add script that calls openssl commands in order to generate self-signed root CA, device key and cert.

Device key and cert are converted to DER format and copied to keys/ subdirectory, so those can be used with following arguments to west build command:

-DCONFIG_GOLIOTH_SYSTEM_CLIENT_CRT_PATH='"keys/PRJ-DEV.crt.der"'
-DCONFIG_GOLIOTH_SYSTEM_CLIENT_KEY_PATH='"keys/PRJ-DEV.key.der"'

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit e0bfc31):

https://golioth-zephyr-sdk-doxygen-dev--pr321-sample-hello-gen-q44rx9pl.web.app

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