make rate limiting whitelist a function so users can override with dynamic behaviour

* Make rate-limiting client whitelist dynamic
* Refactor `RateLimitOptions.ClientWhiteList`
* Fix typo in variable `enbleRateLimiting`
* Fix case in variable `clientIdheader`
author Taiwo Otubamowo <totubamowo@deloitte.co.uk>

* fix 1045

* #492 log 500 with error log level, acceptance test, unit test

* #492 minor changes

* initial commit for new feature #1077

allow to limit the number of concurrent tcp connection to a downstream service

* protect code against value not in accurate range

add unit test

* Do not crash host on Dispose

* Add test

* Pin GitVersion.CommandLine package version

* #683 validate if there are duplicated placeholders in UpstreamPathTemplate

* Use registered scheme from Eureka (#1087)

* extra test

* very brief mention MaxConnectionsPerServer in docs

* build develop like a PR

* more docs

* test

Co-authored-by: Taiwo O. <44668623+totubamowo@users.noreply.github.com>
Co-authored-by: Catcher Wong <catcher_hwq@outlook.com>
Co-authored-by: jlukawska <56401969+jlukawska@users.noreply.github.com>
Co-authored-by: buretjph <58700930+buretjph@users.noreply.github.com>
Co-authored-by: Jonathan Mezach <jonathanmezach@gmail.com>
Co-authored-by: 彭伟 <pengweiqhca@sina.com>

test/Ocelot.UnitTests/Configuration/RateLimitOptionsCreatorTests.cs

@@ -50,7 +50,7 @@ namespace Ocelot.UnitTests.Configuration
             };
             var expected = new RateLimitOptionsBuilder()
                 .WithClientIdHeader("ClientIdHeader")
-                .WithClientWhiteList(fileReRoute.RateLimitOptions.ClientWhitelist)
+                .WithClientWhiteList(() => fileReRoute.RateLimitOptions.ClientWhitelist)
                 .WithDisableRateLimitHeaders(true)
                 .WithEnableRateLimiting(true)
                 .WithHttpStatusCode(200)