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GH-35730: [C++] Add the ability to specify custom schema on a dataset write (#35860)

### Rationale for this change

The dataset write node previously allowed you to specify custom key/value metadata on a write node.  This was added to support saving schema metadata.  However, it doesn't capture field metadata or field nullability.  This PR replaces that capability with the ability to specify a custom schema instead.  The custom schema must have the same number of fields as the input to the write node and each field must have the same type.

### What changes are included in this PR?

Added `custom_schema` to `WriteNodeOptions` and removed `custom_metadata`.

### Are these changes tested?

Yes, I added a new C++ unit test to verify that the custom info is applied to written files.

### Are there any user-facing changes?

No.  Only new functionality (which is user facing)

* Closes: #35730

Lead-authored-by: Weston Pace <weston.pace@gmail.com>
Co-authored-by: Nic Crane <thisisnic@gmail.com>
Co-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Co-authored-by: anjakefala <anja@voltrondata.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Signed-off-by: Weston Pace <weston.pace@gmail.com>