[generator] Separate metadata fixup step from parsing step.

[jpobst]

Context: #789

One issue with the proposal to flip the order of ApiXmlAdjuster and metadata in #789 is that the "apply metadata" step is intertwined with the "parse API xml document into POCO's" step. This means that the adjuster step cannot be inserted between them as the code is currently written.

This PR separates the two steps so that they can be reordered in the future.

Additionally it refactors the metadata fixup step so that it can be unit tested easier by moving it to Java.Interop.Tools.Generator. Unit tests for applying metadata have been added.

[jonpryor]

One possible concern here is that this uses the current culture info to parse value: https://docs.microsoft.com/en-us/dotnet/api/system.int32.tryparse?view=net-5.0#System_Int32_TryParse_System_String_System_Int32__

The s parameter is parsed using the formatting information in a NumberFormatInfo object initialized for the current system culture.

We may want to instead use:

int.TryParse (value, NumberStyles.Integer, CurrentCulture.InvariantCulture, out var result)

[jonpryor]

Alternate idea: why don't we use the explicit conversion from XAttribute to int? operator?

public static int? XGetInt32Attribute (this XElement element, string name) => (int?) element.Attribute (name);

[jpobst]

This seems to throw an exception on invalid input:

System.FormatException
  HResult=0x80131537
  Message=Input string was not in a correct format.
  Source=System.Private.CoreLib
  StackTrace:
   at System.Number.ThrowOverflowOrFormatException(ParsingStatus status, TypeCode type)
   at System.Number.ParseInt32(ReadOnlySpan`1 value, NumberStyles styles, NumberFormatInfo info)
   at System.Int32.Parse(String s, NumberStyles style, IFormatProvider provider)
   at System.Xml.XmlConvert.ToInt32(String s)
   at System.Xml.Linq.XAttribute.op_Explicit(XAttribute attribute)

[jonpryor]

Indentation.

[jonpryor]

I wonder if we should capture elem.XGetAttribute("name") into a name variable, so that we don't call it twice.