Small optimizations to base64decode

[anuraaga]

At an airport so just looking for small stuff :)

• Don't copy input string to bytes since indexing by number indexes bytes
• Use strings.Builder, which doesn't copy to output string

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After

goos: darwin
goarch: arm64
pkg: github.com/corazawaf/coraza/v3/transformations
BenchmarkB64Decode
BenchmarkB64Decode/VGVzdENhc2U=
BenchmarkB64Decode/VGVzdENhc2U=-10         	16657209	        68.79 ns/op
BenchmarkB64Decode/P.HNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==
BenchmarkB64Decode/P.HNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==-10         	183049256	         6.552 ns/op
BenchmarkB64Decode/VGVzdABDYXNl
BenchmarkB64Decode/VGVzdABDYXNl-10                                  	17273186	        68.68 ns/op
PASS

Before

goos: darwin
goarch: arm64
pkg: github.com/corazawaf/coraza/v3/transformations
BenchmarkB64Decode
BenchmarkB64Decode/VGVzdENhc2U=
BenchmarkB64Decode/VGVzdENhc2U=-10         	14609536	        79.49 ns/op
BenchmarkB64Decode/P.HNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==
BenchmarkB64Decode/P.HNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==-10         	32041297	        37.65 ns/op
BenchmarkB64Decode/VGVzdABDYXNl
BenchmarkB64Decode/VGVzdABDYXNl-10                                  	15384277	        76.53 ns/op
PASS