Add CoalescePartitionsExec #1384

Job	Run time
Check cargo fmt	40s
cargo check	2m 9s
build with wasm-pack	2m 12s
cargo test (macos)	9m 16s
cargo test (macos-aarch64)	10m 11s
Verify MSRV (Min Supported Rust Version)	4m 14s
Verify Vendored Code	49s
cargo test (win64)	30m 51s
cargo examples (amd64)	0s
cargo test (amd64)	0s
cargo doc	0s
cargo test datafusion-cli (amd64)	0s
cargo test doc (amd64)	0s
verify benchmark results (amd64)	0s
check Cargo.toml formatting	0s
clippy	0s
check configs.md is up-to-date	0s
cargo test hash collisions (amd64)	0s
Run sqllogictest with Postgres runner	0s
cargo test pyarrow (amd64)	0s
	1h 0m 22s

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