Support external fmt dependency

[kennyweiss]

Axom currently vendors a copy of the fmt library in the <axom>/src/thirdparty/axom/fmt directory and places it under the axom::fmt namespace to avoid clashes with other common libraries that use fmt, e.g. conduit and umpire. This is convenient since fmt is a required dependency for axom and we cannot build the code without it.

In their latest release, umpire began using an external fmt, e.g. built/installed through spack or vcpkg, which has created some issues for axom developers and users in their various build and test configurations. Notably, in #1301, we had to disable Umpire in our Windows build due to ambiguities between the built-in and external fmt in the vcpkg configuration.

To resolve these, axom should support both an externally built/configured fmt, when supplied, and an internal/vendored/submoduled copy otherwise.

Once this is resolved, we can add umpire back to our vcpkg configuration.