feat: Add Fluent response API

[skwasjer]

Per the discussion in #9 this PR adds a new Fluent response API, replacing the current overload based API.

mock.When(_ => {})
    .Respond(with => with
        .StatusCode(200)
        .Body("my text", Encoding.UTF8)
        .ContentType("text/plain")
    );

JSON

mock.When(_ => {})
    .Respond(with => with
        .StatusCode(HttpStatusCode.BadRequest)
        .JsonBody(new { error = "oops" })
    );

Additionally, it adds capabilities to simulate network latency:

mock.When(_ => {})
    .Respond(with => with
        ...
        .Latency(ThreeG)
    );

And network transfer rates (only with streams):

Stream stream = ...; // A big stream

mock.When(_ => {})
    .Respond(with => with
        .Body(new RateLimitedStream(stream, 512000)) // Rate limited to 512 kbps
    );

Furthermore, .TimesOut and TimesOutAfter alternatives have been renamed to .ClientTimeout(). A .ServerTimeout() was also added which instead of throwing a TaskCancelledException after a specified amount of time returns a response with a HTTP 408 Request Timeout instead.

There is still work to be done, notably ensuring all 'old' API's are deprecated. I think I did most but probably there are some gaps left.

One notable issue is the inconsistency with the request API, eg. .Content() and .JsonContent() whereas this new response API uses .Body() and .JsonBody() respectively (still doubting whether or not to just go with .Json(). So probably have to align those too. The downside is more of the API surface changes, making refactoring/adoption harder.

Alpha packages (from this PR) can be fetched from this repo its NuGet feed for testing (it is not published to NuGet until merged):
https://ci.appveyor.com/nuget/mockhttp/packages

[codecov-commenter]

Codecov Report

Merging #10 (0c70251) into main (58b2183) will decrease coverage by 1.10%.
The diff coverage is 82.00%.

❗ Current head 0c70251 differs from pull request most recent head 341ef96. Consider uploading reports for the commit 341ef96 to get more accurate results

Impacted Files	Coverage Δ
...son/Extensions/MockHttpRequestContextExtensions.cs	100.00% <ø> (ø)
src/MockHttp.Json/Newtonsoft/RespondsExtensions.cs	50.00% <ø> (ø)
src/MockHttp.Json/RespondsExtensions.cs	50.00% <25.00%> (-23.92%)	⬇️
src/MockHttp/Responses/HttpContentBehavior.cs	66.66% <66.66%> (ø)
...c/MockHttp/Extensions/ResponseBuilderExtensions.cs	68.91% <68.91%> (ø)
src/MockHttp/Extensions/IRespondsExtensions.cs	84.93% <73.33%> (-10.31%)	⬇️
src/MockHttp/Responses/NetworkLatencyBehavior.cs	75.00% <75.00%> (ø)
src/MockHttp/Extensions/ListExtensions.cs	78.94% <78.94%> (ø)
src/MockHttp/NetworkLatency.cs	83.33% <83.33%> (ø)
...MockHttp/Language/Flow/Response/ResponseBuilder.cs	85.18% <85.18%> (ø)
... and 12 more