Support SM2 two-party threshold signature #6

Job	Run time
address_ub_sanitizer	11m 11s
basic_clang	4m 3s
basic_gcc	4m 10s
check_update	1m 2s
check-ansi	1m 59s
minimal	2m 10s
no-deprecated	3m 55s
non-caching	4m 16s
no-shared (ubuntu-latest)	3m 6s
no-shared (macos-latest)	4m 1s
buildtest	6m 52s
EC_POINTs_api_test	18m 49s
enable_non-default_options	3m 31s
external-test-pyca (1.51.0, 3.9)	1m 24s
fips_and_ktls	4m 17s
legacy	3m 51s
memory_sanitizer	25m 56s
no-legacy	4m 12s
ntls_address_ub_sanitizer	9m 34s
ntls_memory_sanitizer	17m 43s
ntls-with-fips	7m 49s
ntls-without-sm234	6m 40s
out-of-source-and-install (ubuntu-latest)	4m 20s
out-of-source-and-install (macos-latest)	8m 49s
phe_test	36m 44s
symbol_prefix-test	7m 0s
threads_sanitizer	3m 30s
sm2-threshold-test	43m 11s
smtc_and_ntls	14m 9s
zkp-build-test	3m 21s
	4h 31m 35s

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