Introduce fmt dependency to simplify string formatting

[PragmaTwice]

In this PR, we introduce fmt, the de facto standard of string formatting in C++ and the origin of std::format (c++20) & std::print (c++23), which is also the most popular formatting library in C++, used in softwares like
ceph, mariadb, scylladb and mongodb.

And we can replace the old formatting methods (this PR replaced part of them, far from all):

• snprintf or similiar C functions: need to specify types like d, f, s, g, and hard to format basic types like uint64_t (need some ugly macros). cannot format custom types. need to create a fixed-length buffer by our own.
• std::stringstream or similiar stream-like C++ types: very slow, and verbose to write.
• std::to_string and string concat (like "..." + to_string(num1) + "..." + to_string(num2)): inefficient, and hard to write & read.

fmt is using the MIT license with an exemption clause: https://github.com/fmtlib/fmt/blob/master/LICENSE.rst

[torwig]

@PragmaTwice Great Job! It's a really handy and compact way to format strings.

LGTM.

[git-hulk]

@PragmaTwice Changes are good to me, but we need to update the NOTICE as well.

[PragmaTwice]

@PragmaTwice Changes are good to me, but we need to update the NOTICE as well.

Done

[tisonkun]

Merging...

BTW, NOTICE file can be concise. Depencies' licenses and notices can be simply copied under the licenses folder. Then we don't maintain a big text file (manually) :)