GH-48750: [CI][Documentation] Disable Unity build for OpenTelemetry (#48751)

### Rationale for this change

After commit cbd48a4 migrated `OpenTelemetry` from `ExternalProject` to `FetchContent`, the documentation build started failing with symbol redefinition errors:

```
error: redefinition of 'const google::protobuf::internal::MigrationSchema schemas []'
```

`FetchContent` integrates `OpenTelemetry` into the parent CMake configuration, causing it to inherit `CMAKE_UNITY_BUILD=ON`. Unity builds combine multiple `.pb.cc` files into a single compilation unit, which causes collision of `static` symbols (like `schemas[]` and `file_default_instances[]`) that exist in each `protobuf-generated` file.

### What changes are included in this PR?

Added `set(CMAKE_UNITY_BUILD FALSE)` in the `build_opentelemetry()` function in `cpp/cmake_modules/ThirdpartyToolchain.cmake` to disallow Unity build in `protobuf-generated` files.

### Are these changes tested?

Has to be tested via CI.

### Are there any user-facing changes?

No, dev-only.
* GitHub Issue: #48750

Authored-by: Hyukjin Kwon <[email protected]>
Signed-off-by: Raúl Cumplido <[email protected]>

Author: HyukjinKwon

cpp/cmake_modules/ThirdpartyToolchain.cmake

@@ -3726,6 +3726,8 @@ function(build_opentelemetry)
 
   prepare_fetchcontent()
 
+  # Unity build causes symbol redefinition errors in protobuf-generated code
+  set(CMAKE_UNITY_BUILD FALSE)
   set(OTELCPP_PROTO_PATH "${opentelemetry_proto_SOURCE_DIR}")
   set(WITH_EXAMPLES OFF)
   set(WITH_OTLP_HTTP ON)