Include fmt external

[makortel]

The https://github.com/fmtlib/fmt has an implementation of C++20's std::format (and the standard appears to be based on it). Currently no std implementation support std::format, so in the mean time it would be useful to include https://github.com/fmtlib/fmt as an external.

[makortel]

assign core

[cmsbuild]

New categories assigned: core

@Dr15Jones,@smuzaffar,@makortel you have been requested to review this Pull request/Issue and eventually sign? Thanks

[cmsbuild]

A new Issue was created by @makortel Matti Kortelainen.

@Dr15Jones, @silviodonato, @dpiparo, @smuzaffar, @makortel can you please review it and eventually sign/assign? Thanks.

cms-bot commands are listed here

[makortel]

@smuzaffar @mrodozov Could you take a look at this at some point? Thanks.

[mrodozov]

@makortel yes, sorry sometime I miss notifications

[makortel]

No problem, thanks!