Add Base64Url fuzzer

[buyaa-n]

Add Base64UrlFuzzer.

Mostly covered char overloads as byte overloads quite similar and can be covered with Base64Fuzzer

[buyaa-n]

@MihuBot fuzz base64url

[MihuBot]

// Base64UrlFuzzer
System.Exception: Expected=Done Actual=InvalidData
   at DotnetFuzzing.Assert.<Equal>g__Throw|0_0[T](T expected, T actual) in D:\a\runtime-utils\runtime-utils\Runner\runtime\src\libraries\Fuzzing\DotnetFuzzing\Assert.cs:line 18
   at DotnetFuzzing.Assert.Equal[T](T expected, T actual) in D:\a\runtime-utils\runtime-utils\Runner\runtime\src\libraries\Fuzzing\DotnetFuzzing\Assert.cs:line 14
   at DotnetFuzzing.Fuzzers.Base64UrlFuzzer.FuzzTarget(ReadOnlySpan`1 bytes) in D:\a\runtime-utils\runtime-utils\Runner\runtime\src\libraries\Fuzzing\DotnetFuzzing\Fuzzers\Base64UrlFuzzer.cs:line 109
   at DotnetFuzzing.Program.<>c__DisplayClass1_0.<RunFuzzer>b__0(ReadOnlySpan`1 bytes) in D:\a\runtime-utils\runtime-utils\Runner\runtime\src\libraries\Fuzzing\DotnetFuzzing\Program.cs:line 91
   at SharpFuzz.Fuzzer.LibFuzzer.Run(ReadOnlySpanAction action, Boolean ignoreExceptions)
==9180== ERROR: libFuzzer: deadly signal
NOTE: libFuzzer has rudimentary signal handlers.
      Combine libFuzzer with AddressSanitizer or similar for better crash reports.
SUMMARY: libFuzzer: deadly signal
MS: 1 ChangeBinInt-; base unit: 80e1c23d46f468d00555be3fb48866ee6830798a
0x53,0x4d,0x3d,
SM=
artifact_prefix='./'; Test unit written to Base64UrlFuzzer-artifact-1
Base64: U009

• Base64UrlFuzzer-input.bin (3 B)

[buyaa-n]

The input that failed the fuzzer is Base64Url.IsValid("SM=") which supposed to return false, but returning true, the fix is up #106771

[buyaa-n]

Last fuzz run with the bug fix #106771 succeeded, merging