feat(core): Allow custom tracing implementations

[mydea]

And do this for opentelemetry, instead of relying on extra exports.

Also needed to move things around a bit to avoid circular dependency build issues.

Missing is getRootSpan which is still different, I will update this in a follow up to actually use the same non-enumerable property under the hood so we can use the same implementation there!

Reasoning

Until now, we had a core implementation of tracing APIs in @sentry/core. In @sentry/opentelemetry, we had some alternative implementations, which where instead re-exported in @sentry/node - so if you do e.g. this:

import { startSpan } from '@sentry/node';

You'll get the correct, OTEL-powered performance API - great!

However, if any code would internally do e.g. this:

import { getActiveSpan } from '@sentry/core'; // import from core, not node

It would not work, because it would use the non-OTEL-powered API.

This PR addresses this by ensuring we use the same API everywhere when @sentry/node is being used.

[lforst]

Looks great! One thing though: Would you mind going into a little bit more detail in the PR description? I know all the context around this change because we had this convo offline but future readers might not know that. Thanks!

[lforst]

What is this change about?

[mydea]

Aligning the APIs - we talked about removing the finish callback, but it's actually still there in core, we may or may not want to delete it (it's not deprecated so far 😬 ), so I had to add it in the otel implementation as well to make the APIs compatible 😬