migrate from boost::{string_ref,variant,optional} to std::{string_view,variant,optional} once Gaudi is moved to C++17

The following discussion from !474 (merged) should be addressed:

@hegner started a discussion:

can you open a ticket to test this w/ string_view as soon as we moved to C++17 ?

Edited Aug 16, 2021 by Gerhard Raven

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