[Bug]: Abseil ignores user-specified CMAKE_MSVC_RUNTIME_LIBRARY

### Describe the issue

When providing `CMAKE_MSVC_RUNTIME_LIBRARY` either on the CLI or as a cache variable before adding Abseil as a sub project, it has no effect.

This is because Abseil forcibly sets the value of these variables here:
https://github.com/abseil/abseil-cpp/blob/cb9a36d737ceaca2bdb1bc62a20b43ed42ec0ae2/CMakeLists.txt#L61-L68

These should either be changed to cache variables (which always cede to an existing cache variable if one is present, unless using `FORCE` or `INTERNAL`), or the logic should be switched to not run if `CMAKE_MSVC_RUNTIME_LIBRARY` is already defined.

This is semi-mitigated by `ABSL_MSVC_STATIC_RUNTIME` but only if the user intends to use one of Abseil's permutations derived from that variable, so it isn't as flexible.

### Steps to reproduce the problem

Configure abseil via `cmake -DCMAKE_MSVC_RUNTIME_LIBRARY=MultiThreaded -DCMAKE_BUILD_TYPE=Debug -GNinja`.

Observe that any created object files expect to be linked against the debug msvc runtime library, ignoring the user-provided value.

### What version of Abseil are you using?

cb9a36d737ceaca2bdb1bc62a20b43ed42ec0ae2

### What operating system and version are you using?

Windows 11

### What compiler and version are you using?

MSVC 17.14

### What build system are you using?

CMake 3.31

### Additional context

_No response_

The issue is in CMakeLists.txt lines 61-68. First, understand how CMake cache variables work and the precedence of FORCE. Look at the existing logic around ABSL_MSVC_STATIC_RUNTIME. The fix likely involves checking if CMAKE_MSVC_RUNTIME_LIBRARY is already set before setting it, or changing the set() to a cache variable. Test by configuring with the provided command and verifying the runtime library flag in the generated build files.

Written by the indexing model from the issue text.