feat(core): make `sentry` optional

[Grvzard]

This may introduce a breaking change for users who have sentry in their production environment. To make this PR work, #396 has to be merged first.

[Grvzard]

I just checked the commit https://github.com/cloudflare/pingora/commit/ab1b717bf587723c1c537d6549a8f8096f0900d4. There is an oversight causing it to fail to compile in release mode: https://github.com/cloudflare/pingora/blob/ab1b717bf587723c1c537d6549a8f8096f0900d4/pingora-core/src/server/mod.rs#L121-L122 should be

#[cfg(all(not(debug_assertions), feature = "sentry"))]
sentry::capture_error(&e);

You can take a look at my commit for other places that need changes.

[eaufavor]

I believe that the change is fully merged into main now. Thanks