Add ActivityContext parsing APIs

[tarekgh]

Fixes #40046

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Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

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[tarekgh]

@CodeBlanch @cijothomas @lmolkova

[CodeBlanch]

nits: Kind of odd to have W3C in the one comment but not the other. How about...

        /// <param name="traceParent">W3C traceparent header value.</param>
        /// <param name="traceState">W3C tracestate header value.</param>

Personally not a fan of the casing "traceparent" & "tracestate" but that's how they are named in the spec.

For the method summary, might want to refer to them collectively as Trace Context? Eg:

        /// <summary>
        /// Parse W3C Trace Context header values into an ActivityContext object.
        /// </summary>

[tarekgh]

Personally not a fan of the casing "traceparent" & "tracestate" but that's how they are named in the spec.

I prefer to case it as the rest of C# method parameters.

[tarekgh]

Thanks @CodeBlanch I have fixed the comments as you suggested.

[noahfalk]

LGTM 👍

[tarekgh]

The failed tests are not related and tracked. merging this one.

Thanks all for your reviews.