Multiple Filesystem subclasses are missing an override for Equals

Currently the `Filesystem` class contains two overloads for the `Equals` method:

`virtual bool Equals(const FileSystem& other) const = 0;`
`virtual bool Equals(const std::shared_ptr& other) const`

{ return Equals(\*other); }

The second is a trivial call to the first for ease of use. The first method is pure virtual and _must_ be overridden by subclasses. The problem is that overriding a single overload of a method also shadows all other overloads. As a result, it is no longer possible to call the `shared_ptr` version of the method. This appears to be the case for the `SubTreeFileSystem` and the `SlowFileSystem` in `filesystem.h` as well as the `S3FileSystem` in `s3fs.h`. There may be other classes with this problem as well, those are just the ones that I noticed. My guess is that what was intended here is to pull the method into the child class's namespace via a using declaration i.e. add `using FileSystem::Equals` to each child class.

**Reporter**: [Vyas Ramasubramani](https://issues.apache.org/jira/browse/ARROW-18217)

_{**Note**: *This issue was originally created as [ARROW-18217](https://issues.apache.org/jira/browse/ARROW-18217). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start with the Equals overloads in filesystem.h and inspect SubTreeFileSystem and SlowFileSystem, then check S3FileSystem in s3fs.h and other FileSystem subclasses for the same overload-shadowing issue. Verify that each affected subclass exposes the shared_ptr overload while retaining its required comparison override, and run the relevant filesystem checks to confirm the overload is callable.

Written by the indexing model from the issue text.