Feature/c sharp code generation pattern validation issue 3145

[Martelleur]

This is a fix for #3145: C# Code Generation doesn't validate patterns

Proposed solution:
Add RegEx if statement, in NSwag.CodeGeneration.CSharp/Templates/Client.Class.liquid

Before:

{%     for parameter in operation.PathParameters -%}
{%         if parameter.IsNullable == false and parameter.IsRequired -%}
        if ({{ parameter.VariableName }} == null)
            throw new System.ArgumentNullException("{{ parameter.VariableName }}");

{%         endif -%}
{%     endfor -%}

After:

{%     for parameter in operation.PathParameters -%}
{%         if parameter.IsNullable == false and parameter.IsRequired -%}
        if ({{ parameter.VariableName }} == null)
            throw new System.ArgumentNullException("{{ parameter.VariableName }}");

{%         endif -%}
{%         if parameter.Schema.Pattern -%}
        if (!System.Text.RegularExpressions.Regex.IsMatch({{ parameter.VariableName }}, "{{ parameter.Schema.Pattern }}"))
            throw new System.ArgumentException("Parameter '{{ parameter.VariableName }}' does not match the required pattern '{{ parameter.Schema.Pattern }}'.");

{%         endif -%}
{%     endfor -%}