FindBoost: Add Boost 1.73 dependencies

Alexander Grundrequested to merge
Flamefire/cmake:nowide into master

Includes Boost.Nowide new library

Special case: Boost.Filesystem is an optional dependency as the include is only required to make Boost.Filesystem use UTF-8 paths on user request

Backport: release
Topic-rename: FindBoost-1.73

Edited by Brad King

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