breaking: make offline optional to allow building without serde

[CathalMullan]

This ties the offline feature to the macros feature, allowing for builds without serde.

I opted to make the Executor::describe function feature gated on offline. This seemed like the simplest option, assuming it only exists to serve the macro feature. But I'd understand if you'd wanted to go with a different approach.

Outside of the feature cfg additions, I also had to remove the use of serde_urlencoded from sqlx-sqlite, manually encoding the key/values instead.

One issue is that the CI currently doesn't test building without the macros feature at all.

In terms of compile time performance, running LIBSQLITE3_SYS_USE_PKG_CONFIG=1 cargo build --release --no-default-features --features runtime-smol,migrate,all-databases resulted in a drop from 28s to 21s with these changes.

• sqlx-with-serde.html
• sqlx-without-serde.html

Does your PR solve an issue?

Fixes #4070

Is this a breaking change?

Yes, due to altering features at the sqlx level.
Though this will only impact anyone setting default-features = false.

And due to changing the executor sqlx-core API by adding the feature gate.