Preserve tracing headers during FromHTTPHeaders conversion

[kexiongliu123]

RELEASE NOTES:
This change updates headerMapper.FromHTTPHeaders to preserve tracing headers such as uber-trace-id and uberctx-* even when they are not prefixed with Rpc-Header-. These headers are critical for baggage and trace context propagation in services using the new tracing interceptor. Additionally, unit tests in header_test.go are updated to assert correct preservation and round-trip conversion of both application and tracing headers. This ensures that services like Cinnamon can properly extract context even when the outbound tracing interceptor is disabled.

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✅ kexiongliu123
❌ kl943
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[biosvs]

TBH I don't really like that uber-specific stuff is here, but we're using deprecated open source library that has all these constants, so we basically partially merging that library inside a yarpc-go. And eventually we'll get rid of outdated dependency.

(It's not an call-to-action comment, just leaving it here for the history.)

[biosvs]

LGTM, you can move to testing