This adds a new "elasticsearch-keystore show" command that displays
the value of a single secure setting from the keystore.
An optional `-o` (or `--output`) parameter can be used to direct
output to a file.
The `-o` option is required for binary keystore values
because the CLI `Terminal` class does not support writing binary data.
Hence this command:
elasticsearch-keystore show xpack.watcher.encryption_key > watcher.key
would not produce a file with the correct contents.
Co-authored-by: Ioannis Kakavas <ikakavas@protonmail.com>
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