Add type coercion to top-down fix expression types

[max-hoffman]

Setup an outline for how top-down hinting can help type coerce expressions that otherwise lack the context to know what types they need to be. The benefits of this compared to type casting within individual expression Evals:

• Controlling types upfront should aid correctness, for example for aggregation functions that return different types depending on the context, or INSERTs/UPDATEs that expect specific type inputs, or bindvar substitution
• Adding type casts while we have info at the AST layer should generalize to more expression types than adding type casts in individual execution functions
• Front-loading type information should make validation easier to add at binding, which is preferable to after analysis and is probably easier than doing another tree walk

fixes: dolthub/dolt#7018

[jycor]

LGTM

[max-hoffman]

closing this for now, refer here for background: https://docs.google.com/document/d/1AaoIGaoLktCFtQeqJLzJsQMKYL12K1GvnyVTq-WkeTg/edit