Make `cargo sqlx prepare` cache whole db structure, instead of individual queries

[AdrianEddy]

If possible, it will be useful to add a way to prepare/cache the whole database structure (all tables/columns), instead of only used queries. This would allow to have the database mapped once and continue with development without the need to update cached file after each new/changed query in code.

This can be added as additional parameter, for example cargo sqlx prepare full

[abonander]

Caching the schema would not be useful to the macros as they do not analyze SQL themselves; it's necessary to pass the SQL to the database engine for parsing and analysis. If we wanted to analyze the SQL ourselves we would essentially be reimplementing a good chunk of the server frontend for every database flavor we intend to support (since each has a different feature set and different subsets of the SQL standard it supports). Not exactly trivial.

There's discussion about making query data caching more automatic in #570. Otherwise, I'm closing this issue as not currently actionable.