Support custom test metadata

[bhirsz]

After the following PR is merged we need to figure out how to support custom test metadata: robotframework/robotframework#5495

It will allow the following syntax:

Comprehensive Metadata Test
    [Documentation]    Main test case covering essential metadata variations
    [Tags]                         abc    def
    [Owner]                        John Doe    Jane Smith
    [Requirement]                  REQ-001    REQ-002
    [Priority]                     High
    [Component]                    Authentication
    [Feature]                      Login System
    [Build]                        ${BUILD_NUMBER}
    [Long Description]             ${LONG_TEXT}

We will need to update the rules that recognize unknown settings as errors (possibly they will work correctly if they use robot parser).
Formatter should only update known settings. We can handle them in order settings by common marker (ie. 'metadata'). Length, spacing etc rules should support them.
The alignment formatters should be able to align them.