fix: Allow single cert in X.509 credentials #191

Job	Run time
build (i686-pc-windows-msvc, debug)	55s
build (i686-pc-windows-msvc, release)	47s
build (x86_64-pc-windows-msvc, debug)	42s
build (x86_64-pc-windows-msvc, release)	52s
build (x86_64-apple-darwin, debug)	50s
build (x86_64-apple-darwin, release)	1m 0s
build (aarch64-apple-darwin, debug)	49s
build (aarch64-apple-darwin, release)	54s
build (x86_64-unknown-linux-gnu, debug)	46s
build (x86_64-unknown-linux-gnu, release)	48s
build (i686-unknown-linux-gnu, debug)	47s
build (i686-unknown-linux-gnu, release)	50s
build (aarch64-unknown-linux-gnu, debug)	49s
build (aarch64-unknown-linux-gnu, release)	57s
build (aarch64-linux-android, debug)	42s
build (aarch64-linux-android, release)	1m 8s
build (armv7-linux-androideabi, debug)	53s
build (armv7-linux-androideabi, release)	54s
build (x86_64-linux-android, debug)	47s
build (x86_64-linux-android, release)	56s
build (i686-linux-android, debug)	44s
build (i686-linux-android, release)	50s
build (aarch64-apple-ios, debug)	48s
build (aarch64-apple-ios, release)	50s
build (wasm32-unknown-unknown, debug)	48s
build (wasm32-unknown-unknown, release)	55s
build (aarch64-apple-ios-sim, macos-latest)	59s
	23m 0s

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