Add input event callback to `DisplayServerHeadless` #31941

Job	Run time
📊 Static checks / Code style, file formatting, and docs	24s
🍎 macOS / Editor (target=editor, tests=yes)	32m 41s
🍎 macOS / Template (target=template_release)	4m 12s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	3m 43s
🌐 Web / Template w/o threads (target=template_release, threads=no)	4m 0s
🤖 Android / Template (target=template_release)	3m 32s
🍏 iOS / Template (target=template_release)	1m 36s
🐧 Linux / Editor w/ Mono (target=editor)	4m 49s
🏁 Windows / Editor (target=editor, tests=yes)	49m 58s
🐧 Linux / Editor with doubles and GCC sanitizers (target=editor, tests=yes, dev_build=yes, scu_build=yes, precision=double, use_asan=yes, use_ubsan=yes, linker=gold)	37m 36s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	35m 1s
🏁 Windows / Template (target=template_release)	7m 44s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	30m 29s
🐧 Linux / Template w/ Mono (target=template_release)	1m 53s
🐧 Linux / Minimal template (target=template_release, everything disabled)	42s
🪲 Godot CPP / Build and test Godot CPP	7m 25s
	3h 45m 45s

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