[Bug]: kLogMessageBufferSize is not user-configurable

### Describe the issue

kLogMessageBufferSize is set to an arbitrary value that may not work for all use cases

### Steps to reproduce the problem

Log something longer than kLogMessageBufferSize with LOG(...) << ...

### What version of Abseil are you using?

HEAD

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

Doesn't matter (Windows)

### What compiler and version are you using?

Doesn't matter (MSVC)

### What build system are you using?

Doesn't matter (CMake)

### Additional context

_No response_