feat: Allow referencing expressions and constants in policy parameters from other source files in project

[Mielek]

Closes #56, #184

This allows having set of expressions and set of constants which can be reuse as policy parameters.

Limitations:

Reusable constants need to be defined as const.

Reusable expressions will not be inline if used inside other expressions.

This PR does not touch on reusable methods for policy fragments

Example:

Constants.cs

public static class Constants
{
    public const string Username = "{{username}}";
    public const string Password = "{{password}}";
    public const string AzureManagementUrl = "https://management.azure.com/";
}

Expressions.cs

public static class Expressions
{
     public bool IsFromCompanyIp(IExpressionContext context)
        => context.Request.IpAddress.StartsWith("10.0.0.");
}

PolicyDocumentA.cs

[Document]
public class PolicyDocumentA : IDocument
{
    public void Inbound(IInboundContext context) 
    { 
        if (Expressions.IsFromCompanyIp(context.ExpressionContext))
        {
            context.AuthenticationBasic(Constants.Username, Constants.Username);
        }
        else
        {
            context.AuthenticationManagedIdentity(new ManagedIdentityAuthenticationConfig()
            {
                Resource = Constants.AzureManagementUrl,
            });
        }
        // other policies
    }
}

PolicyDocumentB.cs

[Document]
public class PolicyDocumentB : IDocument
{
    public void Inbound(IInboundContext context) 
    { 
        if (Expressions.IsFromCompanyIp(context.ExpressionContext))
        {
            context.AuthenticationBasic(Constants.Username, Constants.Username);
        }
        else
        {
            context.AuthenticationManagedIdentity(new ManagedIdentityAuthenticationConfig()
            {
                Resource = Constants.AzureManagementUrl,
            });
        }
        // other policies
    }
}

After this PR compiling above should produce
PolicyDocumentA.xml

<policies>
  <inbound>
    <choose>
      <when condition="@(context.Request.IpAddress.StartsWith("10.0.0."))">
        <authentication-basic username="{{username}}" password="{{password}}" />
      </when>
      <otherwise>
        <authentication-managed-identity resource="https://management.azure.com/" />
      </otherwise>
    </choose>
    <!-- other policies -->
  <inbound>
<policies>

PolicyDocumentB.xml

<policies>
  <inbound>
    <choose>
      <when condition="@(context.Request.IpAddress.StartsWith("10.0.0."))">
        <authentication-basic username="{{username}}" password="{{password}}" />
      </when>
      <otherwise>
        <authentication-managed-identity resource="https://management.azure.com/" />
      </otherwise>
    </choose>
    <!-- other policies -->
  <inbound>
<policies>