ExcelDnaPack: Add support for packing any kind of resource

[augustoproiete]

Relates to #329.

It would be useful if ExcelDnaPack could be adapted to allow any kind of file to be embedded in the -packed.xll file.

We could include a new element e.g.Resource to the .dna schema that allows the user to specify the file path along with some (optional) metadata (e.g. Type), combined with an API exposed by ExcelDna.Integration that gives the developer access to the resources packed.

e.g.

<?xml version="1.0" encoding="utf-8"?>
<DnaLibrary Name="Add-In" RuntimeVersion="v4.0" xmlns="http://schemas.excel-dna.net/addin/2018/05/dnalibrary">
  <ExternalLibrary Path="AddIn.dll" ExplicitExports="false" LoadFromBytes="true" Pack="true" IncludePdb="true" />
  <!-- ... -->

  <Resource Path="NativeLibrary_32.dll" Type="NATIVELIB" Pack="true" />
  <Resource Path="NativeLibrary_64.dll" Type="NATIVELIB" Pack="true" />

  <Resource Path="Template.xlsx" Type="XLSX" Pack="true" />
</DnaLibrary>

---

public static class ExcelDnaUtil
{
    /// <summary>Returns the names of all the resources packed in the XLL.</summary>
    /// <returns>An enumerable that contains the names of all the resources.</returns>
    public static IEnumerable<string> GetPackedResourceNames() { ... }

    /// <summary>Loads the specified resource from the XLL.</summary>
    /// <param name="name">The case-sensitive name of the resource being requested.</param>
    /// <returns>The resource; or <see langword="null" /> if the resource does not exist in the XLL.</returns>
    public static Stream GetPackedResourceStream(string name) { ... }
}