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GH-38376 [R]: Add `dimnames` method to `Dataset` class  (#38377)

### Rationale for this change

Add `dimnames` method so that things like `colnames(ds)` work out of the box on datasets. Inspired by dbplyr's lazy tbl implementation

### What changes are included in this PR?

An additional `dimnames` method for Dataset objects

### Are these changes tested?

Yes

### Are there any user-facing changes?

Yes
* Closes: #38376

Authored-by: Jonathan Keane <jkeane@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>