[Fuzzing X64] [Copilot] Add Base64 parity APIs with Base64Url

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Job completed in 35 minutes 25 seconds (remote runner delay: 1 minute 17 seconds).

Using arguments: fuzz Base64 -NoPRLink
Main commit: dotnet/runtime@dcbf341
PR commit: dotnet/runtime@8fd3ed6

ERROR: System.Exception: prepare-fuzzer.bat  failed with exit code 1
   at Runner.JobBase.RunProcessAsync(String fileName, String arguments, List`1 output, String logPrefix, String workDir, Boolean checkExitCode, Func`2 processLogs, Boolean suppressOutputLogs, Boolean suppressStartingLog, ProcessPriorityClass priority, List`1 envVars, CancellationToken cancellationToken) in D:\a\runtime-utils\runtime-utils\Runner\JobBase.cs:line 472
   at Runner.Jobs.FuzzLibrariesJob.BuildRuntimeAndPrepareFuzzerAsync() in D:\a\runtime-utils\runtime-utils\Runner\Jobs\FuzzLibrariesJob.cs:line 71
   at Runner.Jobs.FuzzLibrariesJob.RunJobCoreAsync() in D:\a\runtime-utils\runtime-utils\Runner\Jobs\FuzzLibrariesJob.cs:line 34
   at Runner.JobBase.RunJobAsync() in D:\a\runtime-utils\runtime-utils\Runner\JobBase.cs:line 164

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