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GH-39159 [C++]: Try to make Buffer::device_type_ non-optional (#39150)

### Rationale for this change

Buffer should always have a device type. When unspecified, CPU can be assumed.

### What changes are included in this PR?

A change of the member variable type and some adjustments.

### Are these changes tested?

N/A.

### Are there any user-facing changes?

**This PR includes breaking changes to public APIs.**

`Buffer::device_type_` is now a `DeviceAllocationType` instead of a `std::optional<DeviceAllocationType>`.
* Closes: #39159

Authored-by: Felipe Oliveira Carvalho <felipekde@gmail.com>
Signed-off-by: Felipe Oliveira Carvalho <felipekde@gmail.com>