Expose + document the type coercion API publicly

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

We programmatically make Exprs in IOx for specialized filters using the Expr API and want to create a PhysicalExpr. Prior to #3388, the coercion happened in physical exprs this was fine. Now that it happens earlier in the planning, I need a way to call it otherwise I see errors about types not matching in physical exprs

Describe the solution you'd like
I would like a public API that I can use to call the type coercion logic. This will likely help with #3582 as well.

Also I think it would be a good time to start documenting / providing examples of working with the DataFusion Expr APIs as they are quite powerful (e.g the rewrite rules are getting quite sophisticated)

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Part of upgrading in https://github.com/influxdata/influxdb_iox/pull/5792

[alamb]

I plan to make a PR to fix this

[alamb]

The more I think about this, the more I think that coercion is mostly required for exrp simplification -- so I am thinking of ensuring type coercion is run prior to trying to simplify