ast: provider builders for convenient programmatic construction

I have a use case where I'm generating SQL strings, and in general having `sqlparser::ast` is quite good, but the structs are particularly painful to construct programatically. It'd be interesting if the ast structs offered an optional feature that had https://github.com/elastio/bon builders derived to build them.

Maybe somebody will teach me how to safely construct strings that I can then parse (that'd be fine! I just am not generally eager to be doing string formatting to produce sql, particularly given I don't have full control over the identifiers). For now I am generally just doing this formatting, but I don't love it.

Das Issue nennt sqlparser::ast und ein optionales, von bon abgeleitetes Builder-Feature als Einstiegspunkte; untersuche, wie AST-Strukturen und optionale Features derzeit definiert sind. Das Ergebnis sollte ein abgestimmter, nutzbarer Builder-Ansatz für die relevanten AST-Strukturen sein, der den Bedarf reduziert, SQL-Strings manuell zu formatieren.

Vom Indexierungsmodell aus dem Issue-Text verfasst.