[dataimport] introduce a new MetadataGenerator to replace former MetaGenerator with a similar-but-different API that will allow its usage by the massive store (no more entity object manipulation), while making it a bit more flexible for customization by subclasses (experience taken from client code). Related to #10338621

addressbook
basket
blog
book
calendar
comment
company
email
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folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
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