Refactor building schema statements

[simolus3]

This refactors how CREATE VIEW, CREATE TRIGGER and CREATE INDEX statements are constructed in powersync_replace_schema.

The main motivation here is that we want to be able to generate triggers for raw tables as well. At the moment, CREATE TRIGGER statements are built from a string template specialized for the type of trigger and table. To be able to re-use some of that logic for raw tables in the future, this adds the SqlBuffer utility. It contains several methods to write identifiers, string literals and common fragments into a String buffer. In particular, writing into powersync_crud and defining the "header" of CREATE TRIGGER statements has been refactored into common method calls instead of being a large string literal.

This PR doesn't change the statements generated by the core extension (apart from whitespace). It drastically reduces the amount of intermediate String allocations, but I don't expect that to have a meaningful impact on performance.

This also updates Rust to 1.93, to be able to use the standardized fmt::from_fn, allowing us to provide a callback responsible describing how to format a fragment.

[rkistner]

One edge case that this changes is a table that has both local_only: true and insert_only: true. As far as I can see, the previous logic just treated this as local_only, while the new logic treats this as a weird combination of the two, mostly behaving like insert_only.

Ideally we should just disallow specifying both flags, but that could be a breaking change (a user could easily have specified both accidentally before, and it would have kinda worked as a local-only table).

I'd suggest just modifying TableInfoFlags to consider insert_only as false in that case, and adding additional tests to check that case. Then we can consider that case as an error in the next major release.

[simolus3]

while the new logic treats this as a weird combination of the two, mostly behaving like insert_only.

The insert trigger is the only one where it's a combination. For updates and deletes, we'd just skip the trigger for insert-only tables (so that branch would take precedence over the local-only check, which is a change).

I'd suggest just modifying TableInfoFlags to consider insert_only as false in that case, and adding additional tests to check that case

Done 👍