fix use-after-free in extractHeader for multi-value headers

[netliomax25-code]

extractHeader returns a string_view into the local GetAllOfHeaderAsStringResult. For a header sent with more than one value, getAllOfHeaderAsString joins the values into that result's backing string, so the returned view dangles once extractHeader returns and the result is destroyed. The callers in handler_impl.cc copy attacker-controlled headers like x-android-cert through it, reading freed heap memory. Return std::string so the bytes are copied while the backing string is still alive.

[google-oss-prow]

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[google-oss-prow]

Hi @netliomax25-code. Thanks for your PR.

I'm waiting for a GoogleCloudPlatform member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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