utils: implement "lazy" format_locale helper function

[slint]

Since get_locale is actually relying on the request/session/application global state, it could be that we can inject it as a lazy-loaded value in places. We have a utility that injects the locale in format-style string, which for end usable would look like:

from invenio_i18n import format_locale as _l

class AffiliationsSearchOptions(SearchOptions):
    """Search options."""

    suggest_parser_cls = SuggestQueryParser.factory(
        fields=[
            "name^100", 
            _l("title.{locale}^20"),
        ],
    )

    ...

Originally posted by @slint in inveniosoftware/invenio-records-resources#594 (comment)