[C++] Using better namespace style when using protobuf with Substrait

[asfimport]

At the moment the namespace usage in Substrait is not consistent when it comes to using Protobuf classes in serialization and deserialization tasks. 

 

For example: 

Instead of substrait::ReadRel_LocalFiles_FileOrFiles use substrait::ReadRel::LocalFiles::FileOrFiles

 

Reporter: Vibhatha Lakmal Abeykoon / @vibhatha
Assignee: Vibhatha Lakmal Abeykoon / @vibhatha

PRs and other links:

• GitHub Pull Request #14121

_{Note: This issue was originally created as ARROW-17647. Please see the migration documentation for further details.}

[asfimport]

Weston Pace / @westonpace:
Issue resolved by pull request 14121
#14121