PoC Trace Attachments

The SDK should send new trace attachments by adapting the `scope.addAttachment` API ([docs here](https://docs.sentry.io/platforms/javascript/enriching-events/attachments/)) to send the new [trace attachment envelope](https://develop.sentry.dev/sdk/data-model/envelope-items/#trace-attachment).

For now, we'll only experiment with this locally, and perhaps release it in a preview version.

Goals:

* define the new envelope type
* change attachment sending behavior to send an attachment in its standalone envelope once `Scope::addAttachment()`is called.
* test if attachments are ingested and show in the product correctly
* Once initial tests concluded, talk to Telemetry Experience team regarding attachment<>span association ([Span Attachments](https://develop.sentry.dev/sdk/telemetry/spans/span-protocol/#span-attachments)).

Out of Scope:

* merging the PoC into our stable SDK code base (we'll do this in the next major)
* span attachments implementation

Once we go stable (out of scope for this ticket), there should be no user-facing API breaking changes. This *is* still a breaking change, since the new envelope will require a higher self-hosted version.