Fix non-thread-safe use of Random in MultipartFormDataBinaryContent

[stephentoub]

MultipartFormDataBinaryContent has a static Random instance that's being used without a lock. This locks around each access (an alternative would be to use and lazily-initialize on each access a [ThreadStatic] Random).

While fixing that, also improved a few things:

• We don't need to create a char[] for the boundary values; the const string can be used directly.
• For netstandard2.0, we don't need to allocate a char[]; a stackalloc'd span can be used directly.
• For .NET, we don't need a stackalloc, either; we can use string.Create and write directly into the new string instance.
• For .NET, we don't need a dedicated Random instance, either; we can use Random.Shared.
• For .NET, Random allows writing into a byte span, so we don't need the byte[] allocation either and can stackalloc it.
• For .NET, just %'ing by the string length produces as good or better code than using a manually-computed mask.

[KrzysztofCwalina]

cc: @annelo-msft

[annelo-msft]

Adding some context regarding why Random needs to be used in a thread-safe manner: https://devblogs.microsoft.com/pfxteam/getting-random-numbers-in-a-thread-safe-way/