Add ProviderContext.

Adds a ProviderContext type which is passed to LabelProvider#getLabels
and StructuredParameterProvider#getParameter and which carries some
information from ExtLogRecord that is not taken care of by Formatter.

ProviderContext is designed to be extended in the future.

Change-Id: Ib29b7032ae42e0f9e86c75b7404c25cd75b20011
4 files changed