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.