feat: Add BeforeSendTransaction hook

[tonyo]

BeforeSendTransaction works similarly to BeforeSend, but it is applied only to transaction events.

Closes #500.

Docs:

• Basic Options page: feat(go): Add BeforeSendTransaction to Basic Options page sentry-docs#6039
• Filtering page: feat(go): Add BeforeSendTransaction example snippet sentry-docs#6049

[cleptric]

Is there already a docs PR for this?

[cleptric]

Mind adding a small snippet on how to use this?

[tonyo]

I guess adding a link that point to the docs (when they're ready) should be enough?
This change doesn't seem to be too important or breaking enough to cover it in the changelog.

[cleptric]

I would like our changelogs to be a bit more telling, especially when it comes to new features. The Ruby SDK is a good benchmark we should aim for https://github.com/getsentry/sentry-ruby/releases

Maybe we could add the new hook to some example in the repo and point to it?
The docs are not very good at showing the actual use of this.

sentry-go/example/feature-showcase/main.go

Line 116 in 6588be3

func main() {

[tonyo]

Added an example and updated the changelog.
Docs are pending for now, as described in getsentry/sentry-docs#6039