Implement secrets/credentials container

Parent ticket: https://github.com/elastic/logstash/issues/6892

This issue is focused on designing the following:

* [ ] The CLI for storing/purging credentials from the credential store.
* [ ] An API for reading plaintext credentials from the store.
* [ ] A way for plugins to register interest in specific credentials (but not arbitrary credential-store access)

The above could be split into separate issues if desired.

Rough specification:
* The file must be encrypted.
* Humans: Can set and delete, but *not* read, credentials.
* Logstash core should provide a way for Plugins to ask for plaintext values.

Safety goals:
* A passphrase is required in order to read or write to the credential store.
* Plaintext values should never be displayed/logged/printed
* Plaintext values should be only held in memory as long as they are needed.