feat(spanner/spansql): fill in missing hash functions (#4808)

The `spansql` package is quite useful for parsing and managing schemas,
however, with the addition of `CHECK` constraints to the spanner DDL,
the full set of SQL functions can now appear in schemas.

Fill in the missing Hash functions to support their use.

List of hash functions pulled from: https://cloud.google.com/spanner/docs/hash_functions

spanner/spansql/keywords.go

@@ -152,7 +152,9 @@ var allFuncs = []string{
 	"ABS",
 
 	// Hash functions.
+	"FARM_FINGERPRINT",
 	"SHA1",
+	"SHA256", "SHA512",
 
 	// String functions.
 	"BYTE_LENGTH", "CHAR_LENGTH", "CHARACTER_LENGTH",