[Rust][DataFusion] Implement Debug for ExecutionPlan trait

[asfimport]

For ARROW-9653, I was trying to debug the execution plan and I would have found it easier if there had been a way to display the execution plan to better understand and isolate the issue. This would also be nice to have as part of EXPLAIN plan functionality in ARROW-9654

In general, for debugging purposes, we would like to be able to dump out an execution plan. To do so in the idiomatic rust way, we should require that ExecutionPlan also implement std::fmt::Debug

Here is an example plan for "SELECT c1, c2, MIN(c3) FROM aggregate_test_100 GROUP BY c1, c2"

physical plan is HashAggregateExec {
    group_expr: [
        Column {
            name: "c1",
        },
        Column {
            name: "c2",
        },
    ],
    aggr_expr: [
        Min {
            expr: Column {
                name: "c3",
            },
        },
    ],
    input: DataSourceExec {
        schema: Schema {
            fields: [
                Field {
                    name: "c1",
                    data_type: Utf8,
                    nullable: false,
                    dict_id: 0,
                    dict_is_ordered: false,
                },
                Field {
                    name: "c2",
                    data_type: UInt32,
                    nullable: false,
                    dict_id: 0,
                    dict_is_ordered: false,
                },
                Field {
                    name: "c3",
                    data_type: Int8,
                    nullable: false,
                    dict_id: 0,
                    dict_is_ordered: false,
                },
            ],
            metadata: {},
        },
        partitions.len: 1,
    },
    schema: Schema {
        fields: [
            Field {
                name: "c1",
                data_type: Utf8,
                nullable: true,
                dict_id: 0,
                dict_is_ordered: false,
            },
            Field {
                name: "c2",
                data_type: UInt32,
                nullable: true,
                dict_id: 0,
                dict_is_ordered: false,
            },
            Field {
                name: "MIN(c3)",
                data_type: Int64,
                nullable: true,
                dict_id: 0,
                dict_is_ordered: false,
            },
        ],
        metadata: {},
    },
}

Reporter: Andrew Lamb / @alamb
Assignee: Andrew Lamb / @alamb

PRs and other links:

• GitHub Pull Request #7925

_{Note: This issue was originally created as ARROW-9683. Please see the migration documentation for further details.}

[asfimport]

Andy Grove / @andygrove:
Issue resolved by pull request 7925
#7925