Make tokio an optional dependency for the transport feature

[Jasper-Bekkers]

The tokio crate is quite a heavy dependency that we deny in our project through cargo deny, and it seems to be completely optional for sentry as well if we don't compile with the transport feature (which we don't - we provide our own transport layer).

This change make tokio optional for the non-dev dependencies saving on build times.

[Swatinem]

It might be better to couple it to the reqwest/curl/surf features, in line with the mod declaration that pulls this in:

sentry-rust/sentry/src/transports/mod.rs

Lines 11 to 12 in 420444d

	#[cfg(any(feature = "reqwest", feature = "curl", feature = "surf"))]
	mod thread;