Use context for CUDA external memory DMatrix. (#12137) #15103

Job	Run time
Build and test JVM packages (windows-latest)	8m 47s
Build and test JVM packages (macos-15-intel)	9m 35s
Configure variables for CI	10s
Build libxgboost4j.so targeting glibc 2.17 (arch x86_64, runner linux-amd64-cpu)	2m 12s
Build libxgboost4j.so targeting glibc 2.17 (arch aarch64, runner linux-arm64-cpu)	2m 26s
Build libxgboost4j.dylib for MacOS (Intel)	1m 28s
Build libxgboost4j.dylib for MacOS (Apple Silicon)	59s
Build libxgboost4j.so with CUDA	17m 44s
Build and test JVM packages (Linux, Scala 2.13)	5m 20s
Build and test JVM packages (Linux, Scala 2.12)	10m 25s
Test JVM packages with CUDA (Scala 2.13)	8m 48s
Test JVM packages with CUDA (Scala 2.12)	8m 50s
Deploy JVM packages to S3 (gpu, Scala 2.12)	7m 26s
Deploy JVM packages to S3 (gpu, Scala 2.13)	7m 15s
Deploy JVM packages to S3 (cpu, Scala 2.13)	3m 19s
Deploy JVM packages to S3 (cpu, Scala 2.12)	3m 23s
	1h 38m 7s

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