Add hidden CLI arg to control number of shred sigverify threads #10929

Job	Run time
test (token/cli, token/program, token/program-2022, associated-token-account/program, instruction...	19m 57s
cargo-test-sbf (token/program)	8m 53s
test (single-pool/cli, single-pool/program)	18m 28s
check	3m 23s
cargo-test-sbf (instruction-padding/program, token/program-2022, token/program-2022-test)	15m 27s
test (token-upgrade/cli, token-upgrade/program)	18m 0s
cargo-test-sbf (associated-token-account/program, associated-token-account/program-test)	9m 53s
cargo-test-sbf (token-upgrade/program)	8m 39s
cargo-test-sbf (feature-proposal/program)	8m 15s
cargo-test-sbf (governance/addin-mock/program, governance/program)	12m 45s
cargo-test-sbf (memo/program)	9m 2s
cargo-test-sbf (name-service/program)	8m 25s
cargo-test-sbf (stake-pool/program)	11m 47s
cargo-test-sbf (single-pool/program)	9m 48s
	2h 42m 42s

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