[BOLT] Introduce MetadataRewriter interface

Introduce the MetadataRewriter interface to handle updates for various
types of auxiliary data stored in a binary file.

To implement metadata processing using this new interface, all metadata
rewriters should derive from the RewriterBase class and implement
one or more of the following methods, depending on the timing of metadata
read and write operations:

  * preCFGInitializer()
  * postCFGInitializer() // TBD
  * preEmitFinalizer()   // TBD
  * postEmitFinalizer()

By adopting this approach, we aim to simplify the RewriteInstance class
and improve its scalability to accommodate new extensions of file formats,
including various metadata types of the Linux Kernel.

Differential Revision: https://reviews.llvm.org/D154020