feat(sentry-core): add support for w3c traceparent

[saiintbrisson]

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This change does two things:

1. Add a function to allow creating a TransactionContext with a custom Trace ID without a parent span ID. My company is testing traces on Sentry in parallel to our current trace solution, for ease of use, we'd like to keep using the same trace ID on both platforms for now. Currently, my approach is to create a sentry-trace header with a zeroed span ID, but the UI has this bright red alert telling me that the parent span ID is invalid, and this is bugging me. I doubt I'm the only one who has faced this issue before.
2. Implements support for the traceparent as stated in getsentry/team-sdks#41. sentry-trace still takes precedence. It does not emit the header, though. Should it? It only allows for parsing for now.