mbedtls: Add an UT for performance RSA key operations
(New) - Montgomery exponentiation: Z = X ^ Y mod M (HAC 14.94)
keysize = 2048 bits
RSA key operation (performance): public [21894 us], private [199119 us]
RSA key operation (performance): public [18768 us], private [189051 us]
RSA key operation (performance): public [16242 us], private [190821 us]
keysize = 3072 bits
RSA key operation (performance): public [39762 us], private [437480 us]
RSA key operation (performance): public [36550 us], private [449422 us]
RSA key operation (performance): public [40536 us], private [443451 us]
keysize = 4096 bits
RSA key operation (performance): public [65671 us], private [885215 us]
RSA key operation (performance): public [60770 us], private [880936 us]
RSA key operation (performance): public [68951 us], private [872027 us]
(Old) - Sliding-window exponentiation: Z = X ^ Y mod M (HAC 14.85)
keysize = 2048 bits
RSA key operation (performance): public [93206 us], private [280189 us]
RSA key operation (performance): public [93060 us], private [278893 us]
RSA key operation (performance): public [97520 us], private [283252 us]
keysize = 3072 bits
RSA key operation (performance): public [293614 us], private [858157 us]
RSA key operation (performance): public [289902 us], private [843701 us]
RSA key operation (performance): public [291495 us], private [845232 us]
keysize = 4096 bits
RSA key operation (performance): public [653192 us], private [1912126 us]
RSA key operation (performance): public [656661 us], private [1901792 us]
RSA key operation (performance): public [641390 us], private [1938911 us]
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