Add copy constructor for IReadonlyTagHelperAttribute => TagHelperAttribute

[NTaylorMullen]

Today in a TagHelpers Process method you're unable to copy values from TagHelperContext.AllAttributes => TagHelperOutput effectively. Aka this results in an error:

public override void Process(TagHelperContext context, TagHelperOutput output)
{
    output.Attributes.Insert(0, context.AllAttributes[0]);
}

Because context.AllAttributes[0] is an IReadonlyTagHelperAttribute and output.Attributes.Insert only takes TagHelperAttributes (as expected). To make this work you need to re-create the TagHelperAttribute via`

var existingAttribute = context.AllAttributes[0];
var copiedAttribute = new TagHelperAttribute
{
    Name = existingAttribute.Name,
    Value = existingAttribute.Value,
    Minimized = existingAttribute.Minimized
};

It'd be nice to make this slightly easier by providing a semi-copy ctor for TagHelperAttribute so it can take in an IReadonlyTagHelperAttribute and construct itself accordingly. Would let us do:

var existingAttribute = context.AllAttributes[0];
var copiedAttribute = new TagHelperAttribute(existingAttribute);

[dougbu]

Alternatively, could add an Insert(int index, [NotNull] IReadOnlyTagHelperAttribute attribute) or Insert(int index, [NotNull] string name, object value) method to TagHelperAttributeList. The second is a nice match for the existing Add([NotNull] string name, object value) method.

[NTaylorMullen]

Going to take the copy ctor approach since it'd be odd to have Inserts or Adds that accept IReadOnlyTagHelperAttributes (none of the other methods do).