replace StringFormatter with libfmt

[zhjwpku]

Is your feature request related to a problem? Please describe.

fma_common::StringFormatter is easy but lack advanced feature, for example, we can not do the following:

double a = 3.1415926;
StringFormatter.format(buf, "{.2lf}", a);

So I propose replace it with libfmt and formalize the LOG structure.

C++20 has libfmt as a built-in lib, we can easily switch to format after we update to C++20 with few changes.

[zhjwpku]

We have already used the speedlog embeded fmt in v5.x, so close this issue.