create SHA2 UDF

Calculates the SHA-2 family of hash functions (SHA-224, SHA-256, SHA-384, and SHA-512). The first argument is the cleartext string to be hashed. The second argument indicates the desired bit length of the result, which must have a value of 224, 256, 384, 512, or 0 (which is equivalent to 256). If either argument is NULL or the hash length is not one of the permitted values, the return value is NULL.

MySQL also has SHA2 function https://dev.mysql.com/doc/refman/5.5/en/encryption-functions.html#function_sha2