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GH-36483: [C++] Make `UTF8StringToUTF16` and `UTF16StringToUTF8` accept `string_views` (#36485)



### Rationale for this change

As @ kou pointed out in #36366, it's better to pass read-only string data via  as string_views than as const references. I recently submitted two new functions called `UTF8StringToUTF16` and `UTF16StringToUTF8`, both of which accept const references. This PR changes the signatures of those functions to accept string_views.

### What changes are included in this PR?

Changed the function signatures of `UTF8StringToUTF16` and `UTF16StringToUTF8`:

1. `arrow::Result<std::u16string> UTF8StringToUTF16(const std::string& source)` ->
    `arrow::Result<std::u16string> UTF8StringToUTF16(std::string_view source)` 

2. `arrow::Result<std::string> UTF16StringToUTF8(const std::u16string& source)` ->
    `arrow::Result<std::string> UTF16StringToUTF8(std::u16string_view source)` 

### Are these changes tested?

Uses existing tests in `utf8_util_test.cc`

### Are there any user-facing changes?

No.

Only the MATLAB Interface uses these functions, so  I don't expect these changes to break anyone's code:

1. Search [results](https://github.com/search?q=repo%3Aapache%2Farrow%20UTF8StringToUTF16&type=code) for `UTF8StringToUTF16`
2. Search [results](https://github.com/search?q=repo%3Aapache%2Farrow+UTF16StringToUTF8&type=code) for `UTF16StringToUTF8`

* Closes: #36483

Authored-by: Sarah Gilmore <sgilmore@mathworks.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>