[Fuzzing] [buyaa-n] Add fuzzer for Convert.To/FromBase64 APIs

[MihuBot]

Job completed in 2 hours 28 minutes.
dotnet/runtime#108247

Ran the fuzzer(s) successfully.
Artifacts:

• Base64Fuzzer-inputs.zip (863 KB)
• Base64UrlFuzzer-inputs.zip (661 KB)

[MihuBot]

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