Add CoalescePartitionsExec #1392

Job	Run time
Check cargo fmt	49s
cargo check	7m 12s
build with wasm-pack	2m 31s
cargo test (macos)	11m 13s
cargo test (macos-aarch64)	10m 10s
Verify Vendored Code	1m 4s
cargo test (win64)	34m 9s
cargo test (amd64)	9m 54s
cargo test datafusion-cli (amd64)	4m 5s
cargo doc	3m 48s
cargo test doc (amd64)	12m 59s
verify benchmark results (amd64)	11m 24s
Run sqllogictest with Postgres runner	3m 26s
check Cargo.toml formatting	3m 6s
clippy	5m 18s
check configs.md is up-to-date	2m 50s
cargo test pyarrow (amd64)	2m 58s
	2h 6m 56s

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