add missing <optional> header to src/projects/mts/kmer_multiplicity_counter.cpp #123

Job	Run time
🐧 Build SPAdes for Linux	7m 56s
🍎 Build SPAdes for macOS (macos-13, x86_64)	34m 57s
🍎 Build SPAdes for macOS (macos-14, arm64)	8m 54s
SPAdes unittests	22s
🚬 E. coli 1k smoke checks	55s
🚬 E. coli 1k smoke checks (macos-13, x86_64)	2m 23s
🚬 E. coli 1k smoke checks (macos-14, arm64)	1m 8s
SPAdes unittests (macos-13, x86_64)	2m 2s
SPAdes unittests (macos-14, arm64)	55s
	59m 32s

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