Introduce CustomHosts option

GraphClientFactory.md

@@ -18,11 +18,31 @@ There are two primary usage scenarios for the client factory. Developers who wi
  - Set the SdkVersion header with the appropriate moniker based on the following structure `graph-{lang}-{version}`. 
 - Enable the developer to select a supported sovereign cloud using an enumerated list. Selecting the sovereign cloud should ensure that the AuthenticationProvider uses the appropriate Authentication Endpoint.
 - Enable a developer to configure a HTTP proxy that will be used for outgoing requests.
-
+- Enable a developer to provide custom hosts. 
+ - `CustomHosts` option should be set on client creation.
+ - `CustomHosts` option should be made available in the `Context` so that it is available to the middleware during request processing.
+ - These hostnames are different from the Graph service endpoints on the national clouds. 
+ - Certain workloads error out when an unexpected header is present in the request. The middlewares should check: 
+ - If the request URL is a Graph service endpoint or a custom host provided by the developer, then append request headers or modify the request content.
+ - Else the middleware should delete request headers added by that middleware.
+ - Example of an workload error - [LargeFileUploadTask upload to OneDrive caused CORS error due `SDKVersion` telemetry header](https://github.com/microsoftgraph/msgraph-sdk-javascript/issues/265)
+ - Provide capabilities to modify or update the `CustomHosts` option after the client creation. 
+
 ## Performance Considerations
 
 - If available on the platform, enable gzip compression of requests and responses.
 
+## Future Implementation
+
+- Enable developers to set `CustomHosts`. The progress can be tracked as follows:
+```
+| SDK | Implementation Status|
+|-------------|----------------------|
+| C# | - |
+| JAVA | - |
+| PHP | - |
+| JavaScript | In Progress |
+```
 ## Security Considerations
 
 - If available on the platform, configure for TLS 1.2

middleware/AuthorizationHandler.md

@@ -10,6 +10,10 @@ A piece of client-side middleware intended to allow the use of an authorization
 - Transparently authorize requests
 - Accept AuthorizationProvider instance in constructor
 - Update Authorization Header
+- Certain workloads error out when an unexpected header is present in the request. The middlewares should check:
+ - If the request URL is a Graph serve endpoint or a custom url provided by the developer, then append or update the authorization header.
+ - Else the middleware should delete authorization header.
+
 
 Take a request object and use authorization provider to add Authorization header. 
 

middleware/TelemetryHandler.md

@@ -36,6 +36,10 @@ Provide a mandatory middleware component that attaches metadata to a Graph reque
  - `RuntimeEnvironment: Node/1.1` for JavaScript.
  - `RuntimeEnvironment: JRE/1.1` for Java.
 
+- Certain workloads error out when an unexpected header is present in the request. The middlewares should check:
+ - If the request URL is a Graph serve endpoint or a custom url provided by the developer, then append or update the telemetry headers.
+ - Else the middleware should delete telemetry header.
+
 #### Ideal Metadata Capture
 ```
 SdkVersion: graph-dotnet-beta/0.6.0-preview, graph-dotnet-core/1.16.0 (featureUsage=0f)