Add fuzzer for Convert.To/FromBase64 APIs

[buyaa-n]

Add fuzzer for Convert.To/FromBase64*** APIs

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[MihaZupan]

This isn't covering that much logic, and the interesting inputs are the same as with Base64Fuzzer. Can we move the extra asserts into that one instead?

[buyaa-n]

Only Base64FormattingOptions.None path uses Base64 encoder, it has its own logic for other cases and also has logic for widening encoded char after calling Base64.EncodeTo***(...).

i.e there is some logic we want to be covered, plus I assume other Conver APIs fuzz tests will be added to this one

[MihaZupan]

But the inputs are base64, so the interesting corner cases will be similar between base64 APIs on Convert, or on the Base64 class.

If we were to add more fuzz testing for other APIs on Convert, I don't think we'd want them to share the fuzz target with this one as the inputs for base64 parsing have nothing in common with interesting inputs for parsing an int32.

[lewing]

/azp run runtime

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[buyaa-n]

@MihuBot fuzz ConvertToBase64

[MihaZupan]

Fuzzing builds are currently broken as of #106599 being merged (+1 for #107204).

[buyaa-n]

Fuzzing builds are currently broken as of #106599 being merged (+1 for #107204).

Yes, the MihaBot failed with that issue, I am trying to fix that