API Proposal: Add ReadOnlySpan<T> overload for List<T> AddRange and InsertRange

[rabbitism]

public void AddRange(ReadOnlySpan<T> span)
{
}

public void InsertRange(int index, ReadOnlySpan<T> span)
{
}

By Adding this API, it eliminates one extra step of copying span elements into a new array.

[eerhardt]

Looks like a duplicate of https://github.com/dotnet/corefx/issues/36198.

[safern]

Closing as dupe of: #1530

@rabbitism if you're interested on this, please add your motivations on that issue to push it through.