Refactor how we write support file to disk (Registry, Keystore)

[ph]

Currently beats only write state with the registry with the addition of the Keystore in #5687 we will have another way to write content to disk. We should refactor the writing to disk to a common abstraction that can be used in both context.

A few things that need to be supported:

• Write to a temporary file
• Replace an existing file
• If something go wrong abort and keep the original file in place.
• Fix any corner case on shutdown

Related code:

• https://github.com/elastic/beats/blob/master/filebeat/registrar/registrar.go#L230
• https://github.com/elastic/beats/blob/master/filebeat/registrar/registrar.go#L251
• Allow user to locally obfuscate secret in a keystore #5687 (diff)

[kvch]

Issue is tracked here as well: #12908