pac modelbuilder - static const Fields class should be partial

[daryllabar]

I'm on a project that has a very "unique" ALM process such that there is no defined server where the most up to date metadata exists.
If the static fields class would be generated as partial, it would allow for devs to add missing fields that won't get overwritten the next time the classes are regenerated.

Current:

	[System.Runtime.Serialization.DataContractAttribute()]
	[Microsoft.Xrm.Sdk.Client.EntityLogicalNameAttribute("contact")]
	public partial class Contact : Microsoft.Xrm.Sdk.Entity
	{
		
		/// <summary>
		/// Available fields, a the time of codegen, for the contact entity
		/// </summary>
		public static class Fields
		{
			public const string AccountId = "accountid";
			public const string AccountRoleCode = "accountrolecode";
			public const string Address1_AddressId = "address1_addressid";
			public const string Address1_AddressTypeCode = "address1_addresstypecode";
			public const string Address1_City = "address1_city";

Desired

	[System.Runtime.Serialization.DataContractAttribute()]
	[Microsoft.Xrm.Sdk.Client.EntityLogicalNameAttribute("contact")]
	public partial class Contact : Microsoft.Xrm.Sdk.Entity
	{
		
		/// <summary>
		/// Available fields, a the time of codegen, for the contact entity
		/// </summary>
		public static partial class Fields
		{
			public const string AccountId = "accountid";
			public const string AccountRoleCode = "accountrolecode";
			public const string Address1_AddressId = "address1_addressid";
			public const string Address1_AddressTypeCode = "address1_addresstypecode";
			public const string Address1_City = "address1_city";