reactor: Mark _config const #40

Job	Run time
Test with C++20 modules enabled / test	11m 8s
Test (clang++, C++20, debug) / test	13m 21s
Test with DPDK enabled / test	22m 29s
Test (clang++, C++20, dev) / test	12m 17s
Test (clang++, C++20, release) / test	13m 3s
Test (clang++, C++23, dev) / test	15m 45s
Test (clang++, C++23, release) / test	15m 14s
Test (clang++, C++23, debug) / test	15m 39s
Test (g++, C++20, debug) / test	16m 39s
Test (g++, C++20, dev) / test	11m 2s
Test (g++, C++20, release) / test	14m 18s
Test (g++, C++23, debug) / test	18m 32s
Test (g++, C++23, dev) / test	12m 43s
Test (g++, C++23, release) / test	16m 57s
	3h 29m 7s

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