fix: "tracestate" parsing must allow whitespace around list-members

[trentm]

Per https://w3c.github.io/trace-context/#tracestate-header-field-values
the tracestate header can have whitespace around list-members. E.g.
'vendor1=foo, vendor2=bar ,\t vendor3=baz '

Refs: elastic/apm-agent-rum-js#1334

---

I'm fairly confident on the regex changes, but not at all on possible wider impact. I haven't followed closely how _parse_tracestate is used in the agent code.

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• Start Time: 2023-01-27T22:18:15.936+0000

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[basepi]

@trentm it won't let me ask you for review since you're the author...what do you think of my changes?

[trentm]

@basepi Should I merge this? Or should there be a changelog entry or something?
I'm away tomorrow, so feel free to take this one over.