Datanode provisioning refactoring

[todvora]

This PR completely refactors the provisioning and PKI infrastructure in the datanode and partially in the graylog server. Most significant changes:

• Datanode private key and certificates always stored in a standard java keystore on filesystem (not in mongodb anymore)
• Every datanode has always a private key. If not, it will be created during initial boot
• DatanodeKeystore as a central class that manages the keystore
• Certificate signing handling as cluster events: datanode sends a CSR, gets a signed cert chain back
• Removed signing periodicals on both sides
• OpensearchConfigurationProvider now uses events to inform components about configuration changes (inverted workflow)

/nocl

Description

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.