ref: Move trace context initializers to private API

[github-actions]

Note

The pull request "ref: Move trace context initializers to private API" was created by @noahsmartin but did not reference an issue. Therefore this issue was created for better visibility in external tools like Linear.

This initializer doesn't seem to be necessary as part of the public API. This class is public so the values can be read from SentrySpan, but I don't think users of the SDK ever initialize the type. So this PR moves the initializers to a private header

#skip-changelog

[linear]

COCOA-900