MINOR: [R] cast() documented but not exported in NAMESPACE

[AnkitAhlawat7742]

FIX #49852

Rationale for this change

cast() is a publicly documented function available at https://arrow.apache.org/docs/r/reference/cast.html and is intended for use in dplyr pipelines. However, it was marked with @Keywords internal which prevented it from being exported in the NAMESPACE file.

What changes are included in this PR?

Removed @Keywords internal tag which was incorrectly marking cast() as an internal function
Added @export tag so that devtools::document() adds cast to the NAMESPACE file, making it publicly accessible to users

Are these changes tested?

Existing tests in the test suite already cover cast(). No new tests needed as only the export tag was changed, not the implementation.

Are there any user-facing changes?

Yes. cast() can now be called directly by users as documented:

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