Minor: Add `MemoryReservation::consumer` getter #19686

Job	Run time
cargo check	4m 0s
build with wasm-pack	2m 43s
cargo test (win64)	23m 35s
Check cargo fmt	45s
cargo test (mac)	12m 29s
Verify MSRV	12m 9s
Verify Vendored Code	1m 6s
cargo examples (amd64)	6m 52s
cargo test datafusion-cli (amd64)	5m 1s
cargo doc	5m 58s
clippy	7m 1s
cargo test (amd64)	14m 40s
cargo test doc (amd64)	18m 44s
Run sqllogictest with Postgres runner	5m 13s
cargo test pyarrow (amd64)	4m 21s
verify benchmark results (amd64)	17m 3s
check Cargo.toml formatting	1m 54s
check configs.md is up-to-date	4m 1s
cargo test hash collisions (amd64)	13m 41s
	2h 41m 16s

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