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GH-41358: [R] Support join "na_matches" argument (#41372)

### Rationale for this change

Noticed in #41350, I made #41358 to implement this in C++, but it turns
out the option was there, just buried a bit.

### What changes are included in this PR?

`na_matches` is mapped through to the `key_cmp` field in
`HashJoinNodeOptions`. Acero supports having a different value for this
for each of the join keys, but dplyr does not, so I kept it constant for
all key columns to match the dplyr behavior.

### Are these changes tested?

Yes

### Are there any user-facing changes?

Yes
* GitHub Issue: #41358