Cherry-pick #18748 to 7.x: Add initial support for configurable file identity tracking

[kvch]

Cherry-pick of PR #18748 to 7.x branch. Original message:

What does this PR do?

This PR adds a new option to the log input of Filebeat named file_identity. The option lets users configure file identity for state tracking.

Available strategies

1. native (default): Filebeat identifies files based on their inode and device id.
2. path: Files are considered different if they have different paths.
3. inode_marker: A special marker file and the inode is used to tell apart files. It is not supported on Windows.

State IDs previously were not saved to the registry file. Now these are persisted on the disk.

Architecture

I introduced a new interface: file.StateIdentifier. The responsibility of StateIdentifier is to generate an identifier for a file.State based on the configuration. If someone wants to implement their own StateIdentifier method, all they need is to create a struct which satisfies this interface.

// StateIdentifier generates an ID for a State.
type StateIdentifier interface {
	// GenerateID generates and returns the ID of the state
	GenerateID(State) (stateId, identifierType string)
}

As every state has an ID, Filebeat just compares the IDs of the two states to decide if they belong to the same file or not.

The scope of the PR does not include strategies which include fingerprinting the contents of the file.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made the corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[elasticmachine]

Pinging @elastic/integrations-services (Team:Services)