THRIFT-6219: ci(cpp): Check formatting of changed C++ lines only

[slachiewicz]

Adds a CI job that runs clang-format 18 on the lines a pull request changes in lib/cpp, so .clang-format is enforced from now on without reformatting existing code.

Six options are added to .clang-format so that it describes lib/cpp as it is written: FixNamespaceComments, ReflowComments, SortIncludes, BinPackArguments, SortUsingDeclarations and SpacesInLineCommentPrefix. The Base64 decode table is excluded with // clang-format off. With both, formatting all of lib/cpp/src changes 104 files and 1,583 lines, down from 149 files and 2,674. The remainder are files with a local style of their own (indented #if, nested namespaces on one line, aligned enum values) that no global option matches.

The job does not cover compiler/cpp or test/cpp.

Verified: git-clang-format-18 --diff HEAD^ -- lib/cpp exits 1 on a misformatted line in a .tcc file, and 0 on a correctly formatted change and on a change without C++ files.

• Did you create an Apache Jira ticket? (Request account here, not required for trivial changes)
• If a ticket exists: Does your pull request title follow the pattern "THRIFT-NNNN: describe my issue"?
• Did you squash your changes to a single commit? (not required, but preferred)
• Did you do your best to avoid breaking changes? If one was needed, did you label the Jira ticket with "Breaking-Change"?
• If your change does not involve any code, include [skip ci] anywhere in the commit message to free up build resources.

Generated-by: Claude Opus 5.5

[slachiewicz]

@Jens-G, is this the direction you want for THRIFT-6219? It takes the ticket's second option, adjusting .clang-format to the code, and checks only changed lines instead of reformatting lib/cpp once. If you would rather do the one-time reformat, the tuned config makes that commit 104 files instead of 149, and the job can then check whole files.