SQLite extension loading via sqlx.toml for CLI and query macros

[djarb]

fixes issue #3330 by allowing the query checking macros to load SQLite extensions specified in sqlx.toml.

Related to that, also allows the CLI to load the SQLite extensions specified in sqlx.toml when running migrations

[abonander]

I'm very keen in landing this, but besides fixing the build, it does also need some sort of test to prove that it works, and ideally a compile-checked example that shows how to use it. In #3383 I realized I could take out two birds with one stone by adding some example projects showing off the new configuration. I'd suggest doing the same.

[abonander]

(I'm just now realizing the build failures are from #3383 itself so disregard that.)

[djarb]

I'll be happy to add an example, but there's a complication: to demonstrate this functionality, it's necessary to load a SQLite extension, and that requires that the example is dependent on having that extension installed. Are you okay with having an example that has a third-party dependency which is not shipped with SQLx? If so, what is your preference for how I structure that?

[abonander]

Our CI already downloads an extension from https://github.com/nalgeon/sqlean/ for testing:

sqlx/.github/workflows/sqlx.yml

Line 125 in 65229f7

- run: mkdir /tmp/sqlite3-lib && wget -O /tmp/sqlite3-lib/ipaddr.so https://github.com/nalgeon/sqlean/releases/download/0.15.2/ipaddr.so

One thing is that for Unix platforms, you need to show the user either how to set LD_LIBRARY_PATH or to specify a relative path to the extension.