[vcpkg] Rewrite custom fmt formatters to a supported way

[xvitaly]

Describe the bug
Currently, vcpkg uses an unsupported way of creating custom formatters by changing the internal inline namespace, which causes serious problems when building vcpkg against different versions of fmtlib.

The problem is the use of the inline namespace fmt::vX in include/vcpkg/base/fwd/format.h. All other sources are okay and can be built with fmt 8.0 - 10.

I asked upstream and they suggested a good documented way: https://fmt.dev/latest/api.html#formatting-user-defined-types

Environment

• OS: ANY
• Compiler: ANY

To Reproduce
Steps to reproduce the behavior:

1. Try to build against different fmtlib version.

Expected behavior
It should be built successfully.

Actual behaviour
Build failure due to hardcoded inline namespace ABI version.

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