Adding utility methods for parsing webhooks

[slorello89]

ASP.NET / ASP.NET Core doesn't do the best job of handling the webhooks inbound from Vonage. This is mostly because we use - and _ characters in our webhook structs. This update is adding a utility library to allow users to parse webhooks easily from the common endpoints they would be using.

For reference the two main entry points I see are:

1. ASP.NET Core MVC/web API Controllers
2. ASP.NET Classic Web API Controllers

We support 3 content types for our webhooks as best I can gather

1. Queries - this is actually not technically a content type - but it's also the default state your account is put in when you create it. I've added the methods ParseQuery to handle the ASP.NET Core MVC Request.Query property inbound to the controller. I've also added ParseQueryNameValuePairs to handle the Legacy Request.GetQueryNameValuePairs() from ASP.NET Classic
2. URL Form Content
3. JSON Content

in the case of both 2 and 3 I've added ParseWebhook methods that access the HttpRequestMessage (from the legacy ASP.NET implementation) as well as a raw stream and content type for the ASP.NET Core implementation. As both methods access the underlying stream from the request I've added async implementations for them as well.

[codecov-commenter]

Codecov Report

Merging #208 into master will increase coverage by 0.06%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master     #208      +/-   ##
==========================================
+ Coverage   88.02%   88.08%   +0.06%     
==========================================
  Files         211      213       +2     
  Lines        6486     6606     +120     
  Branches      596      612      +16     
==========================================
+ Hits         5709     5819     +110     
- Misses        690      692       +2     
- Partials       87       95       +8     

Impacted Files	Coverage Δ
Nexmo.Api/Utility/WebhookParser.cs	88.88% <88.88%> (ø)
Nexmo.Api.Test.Unit/WebhookParserTests.cs	93.93% <93.93%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e506f2f...ce00e8e. Read the comment docs.

[dragonmantank]

Overall this looks great, it will go a long way toward making the incoming webhook data much more consumable.

Can we add documentation for how the user would implement this, especially since this would be beneficial in generic webapp controllers?

[dragonmantank]

LGTM 👍