[Rust][DataFusion] Add FORMAT to explain plan and an easy to visualize format

[alamb]

Note: migrated from original JIRA: https://issues.apache.org/jira/browse/ARROW-9746

As suggested by [~jorgecarleitao] on apache/arrow#7959 (review) --

Other formats?

One thing I dislike about spark is that EXPLAIN is in a not-so-nice format to parse programmatically. One idea is to allow explain to return the result in json. This is e.g. useful to construct a visual graph representation of the plan. One idea would be to support another string parameter, format, and make it default to string (not necessarily in this PR).

The typical thing I have used in the past to great effect is graphviz: https://graphviz.org/ and dot -- so the idea is that we would get a command like:

{code}
EXPLAIN FORMAT GRAPHVIZ SELECT * FROM FOO
{code}

which would provide the plan in graphviz format (in addition to text)

[alamb]

Comment from Andy Grove(andygrove) @ 2020-08-21T22:52:56.140+0000:

I am also a fan of dot and graphviz. Would be great to see this.

[alamb]

wrong repo