Add Random.Get{Hex}String

[stephentoub]

Fixes #111956

I chose not to adopt RandomNumberGenerator's much more complex GetHexString implementation and instead went with a simple wrapper for GetString. We can evaluate based on perf testing whether it'd be worthwhile to do so, but my expectation is the uses for GetHexString will involve shorter sequences where the perf differential will not huge.

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[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

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