Implement Span Links

[Lms24]

Description

As spec'd out in the RFC (PR of RFC), the JS SDK should support adding and sending span links to Sentry.

This means:

• implement Span.addLink
• implement Span.addLinks
• add links property to StartSpanOptions
• serialize span links in transaction events

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Span Links

• feat(core): Create types and utilities for span links #15375
• feat(opentelemetry): Add addLink(s) to span #15387
• feat(core): Add addLink(s) to Sentry span #15452
• feat(opentelemetry): Add links to span options #15403
• feat(core): Add links to span options #15453

Docs

• docs(span-links): Add develop docs for span links sentry-docs#12829

[Lms24]

Closing as completed. (I removed the ref to #14992 as it is already tracked separately in https://github.com/getsentry/projects/issues/268)