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GH-36227: [C++] New GcsOption to set the project id (#36228)

### Rationale for this change

This fixes #36227, originally motivated by the problems in #36119, but seems like a valuable feature in any case.

### What changes are included in this PR?

- Refactor some code to make it testable.
- Add a new `std::optional<std::string>` field to the `GcsOptions` class.

### Are these changes tested?

Yes, I expanded the unit tests.

### Are there any user-facing changes?

Yes. I updated the field documentation.  If I missed some documentation please let me know.

I am also not familiar with the steps required to update the Python wrappers, if there is some documentation to follow I would appreciate it.  I can expand this PR or send a separate one, your call.

* Closes: #36227

Authored-by: Carlos O'Ryan <coryan@google.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>