[Meta] Keystore and secure settings

[ph]

With the creation of the keystore to obfuscate the secrets in the configuration we still have few things that can be done:

Before 6.2

• Check the permissions when filebeat is installed from packages, see elastic/elasticsearch@7a035f5 our beats run as root.

After 6.2

• Allow user to use their custom password on the keystore Keystore: Allow user to set their own password on the keystore #5737
• Allow the Keystore to be completely pluggable Allow the Keystore to be completely pluggable #5832
• When we have more than one implementation of the Keystore move the the FileKeystore into his own package.
• Refactor the config handling to use the SecureString type to hide the secrets until we really need them.
• Refactor the writing to disk of file see Refactor how we write support file to disk (Registry, Keystore) #5755