Optimize `upb_EpsCopyInputStream` capturing.

Previously, the `capture_start` pointer was stored as a field inside the `upb_EpsCopyInputStream` struct. This structure forced a memory round-trip for writing and reading the start pointer (`e->capture_start`). By moving the tracking pointer to `struct upb_EpsCopyCapture`, callers can maintain the capture context stack-locally. The compiler is now able to allocate this value in a register when the capture is a local variable.

PiperOrigin-RevId: 915664238