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GH-38309: [C++] build filesystems as separate modules (#39067)



### Rationale for this change

Each filesystem implementation carries unique and potentially heavy dependencies, so it'd be useful to build them separately. Furthermore, one typically doesn't need all of them at the same time and building separate modules would allow them to be dynamically loaded as necessary. Finally, defining this interface allows custom filesystem implementations to be supported seamlessly.

### What changes are included in this PR?

An initial sketch of a registry, with documentation as if the registry were complete to illustrate intended usage.

### Are these changes tested?

A toy module is added and a single unit test too.

### Are there any user-facing changes?

Users would be able to add their own filesystem implementations to the registry

* Closes: #38309

Authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>