What does this PR do?

Implements Trace Continuation Strategy per https://github.com/elastic/apm/blob/main/specs/agents/trace-continuation.md

Note for reviewers. The choice of Transaction.onTransactionStart() as the sole location to apply the strategy is because

1. The headers are processed in TraceContext, so would be the ideal place to apply the strategy, but it doesn't have access to
   the span for adding span links, so the options were to
   1a. Reimplement all the trace context header handling to enable that or
   1b. Move up the stack - which is what I chose
2. Moving up the stack, I analyzed all the callers of TraceConext.getFromTraceContextTextHeaders() and getFromTraceContextBinaryHeaders(). These were either in tests (ignorable), or from plugins adding spanlinks (ignorable). Of the remaining two
   2a ExternalSpanContextInstrumentation.parseTextMap is for reading headers only, so can be ignored
   2b JmsInstrumentationHelper.makeChildOf which uses JMS headers so I've assumed that the strategy doesn't apply here (THIS MAY BE AN INCORRECT ASSUMPTION)
3. Consequently places in Transaction which call those two TraceContext methods both funnel through Transaction.onTransactionStart() so this handles both binary and text headers

Checklist

• This is an enhancement of existing features, or a new feature in existing plugins
  • I have updated CHANGELOG.asciidoc
  • I have added tests that prove my fix is effective or that my feature works
  • Added an API method or config option? Document in which version this will be introduced
  • I have made corresponding changes to the documentation
• This is a bugfix
  • I have updated CHANGELOG.asciidoc
  • I have added tests that would fail without this fix
• This is a new plugin
  • I have updated CHANGELOG.asciidoc
  • My code follows the style guidelines of this project
  • I have made corresponding changes to the documentation
  • I have added tests that prove my fix is effective or that my feature works
  • New and existing unit tests pass locally with my changes
  • I have updated supported-technologies.asciidoc
  • Added an API method or config option? Document in which version this will be introduced
  • Added an instrumentation plugin? Describe how you made sure that old, non-supported versions are not instrumented by accident.
• This is something else
  • I have updated CHANGELOG.asciidoc