[C++] `DictionaryBuilder::InsertMemoValues` should not deduplicate values

### Describe the bug, including details regarding any error messages, version, and platform.

Per #47134, duplicate values are tolerable in dictionaries, however the C++ API currently deduplicates memo values even when they are explicitly inserted by the user.

I believe this is the offending line: https://github.com/apache/arrow/blob/main/cpp/src/arrow/array/builder_dict.cc#L85

This becomes an issue when the user thinks they insert N values, but the dictionary ends up containing << N dictionary values. Since the user explicitly called `InsertMemoValues`, they might proceed by directly calling `AppendIndices` in the future - unaware that the dictionary contents do not directly map to the indices the user is appending

### Component(s)

C++