Allow substrait.Plan as an input

[wackywendell]

The current substrait-validator parse interface accepts a Buf object, but there is no equivalent function like validate that accepts an already decoded Plan object and validates it.

There is the parse_plan function, but (1) it's not public (it's pub in the parse module, but the parse module itself is not public), and (2) the external interface should probably be something a bit different, more like fn validate(plan: &Plan, config: &Config) -> ParseResult to match the public parse function.

Out of curiosity - is there any reason this isn't exposed? The one I can guess at is that it would expose the version of substrait that substrait-validator is depending on, and the caller would have to use the same one (or their Plan types wouldn't be the same type); that would limit the utility of this function, but still seems to me like it would be a nice complement to the parse function. Are there other reasons, or is this a good idea?

[wackywendell]

Note: this looks like it has some similarities to #33, although that is Python-focused, and this issue is focused on the Rust API.

[wackywendell]

#281 was merged, so calling this a success.