Add temporary global implementation for logging

[MrAlias]

Building out logging bridges will require some global logging package to reference similar to what exists when the log package becomes stable. Similar to what we did for the metric signal, add a log/global package that implements a global logging provider.

• This package is not expected to be released as stable
  • It will be deprecated and removed when the log package stabilizes
• This package will be copied to otel/internal/global when log stabilizes and appropriate functions added to otel