Add SSL_CTX_add_client_custom_ext (OpenSSL-compat client custom extensions) #17

Job	Run time
Windows MASM AES-GCM-SIV (x64)	1m 37s
Cross-arch test	4m 7s
make check (--enable-cryptonly --enable-aesgcm-siv CPPFLAGS=-DGCM_SMALL)	31s
make check (--enable-cryptonly --enable-aesgcm-siv --enable-smallstack)	32s
make check (--enable-cryptonly --enable-intelasm --enable-sp-asm --enable-aesgcm-siv)	35s
make check (--enable-cryptonly --enable-aesgcm-siv CPPFLAGS=-DGCM_WORD32)	31s
make check (--enable-cryptonly --enable-all-crypto --enable-intelasm --enable-sp-asm --enable-aesgcm-siv)	1m 3s
make check (--enable-cryptonly --enable-aesgcm-siv)	37s
	9m 33s

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