Add `ColumnDef::get_column_default` getter #4445

Job	Run time
Rustfmt	15s
Clippy	1m 11s
Init	4s
Examples (examples/actix_example)	2m 27s
Examples (examples/axum_example)	2m 32s
Unit Test	3m 17s
Examples (examples/basic)	1m 11s
Examples (examples/graphql_example)	2m 31s
Examples (examples/jsonrpsee_example)	2m 41s
Examples (examples/poem_example)	2m 28s
Examples (examples/proxy_gluesql_example)	1m 27s
Examples (examples/rocket_example)	3m 8s
Examples (examples/rocket_okapi_example)	3m 26s
Examples (examples/salvo_example)	2m 45s
Examples (examples/seaography_example)	2m 36s
Examples (examples/tonic_example)	2m 39s
Compile SQLite (async-std, native-tls)	2m 51s
Compile MySQL (actix, native-tls)	2m 54s
CLI	0s
Compile SQLite (async-std, rustls)	2m 58s
Compile MySQL (actix, rustls)	3m 0s
Compile PostgreSQL (tokio, native-tls)	2m 56s
Compile PostgreSQL (tokio, rustls)	3m 8s
Issues Matrix	0s
Issues	0s
MySQL (lts, actix, native-tls)	3m 33s
MariaDB (lts, actix, native-tls)	3m 6s
MySQL (5.7, actix, native-tls)	3m 22s
Postgres (14, tokio, native-tls)	6m 41s
Postgres (16, tokio, native-tls)	6m 49s
SQLite (async-std, native-tls)	4m 33s
SQLite (async-std, rustls)	4m 42s
	1h 25m 11s

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