[core-http] Remove the FetchHttpClient

[sadasant]

The FetchHttpClient was used as the base abstract class for the DefaultHttpClient, however it wasn’t exposed until this very recent PR: #18755

This PR looks to remove the FetchHttpClient.

[xirzec]

I love this change, just one minor comment

[xirzec]

do we still need this? I'm curious how the browser is pulling in NodeFetchHttpClient

[sadasant]

ohh probably not. I will confirm in a bit!

[azure-sdk]

API changes have been detected in @azure/core-http. You can review API changes here

API changes

- export declare class NodeFetchHttpClient extends FetchHttpClient {
+ export declare class NodeFetchHttpClient implements HttpClient {
+     sendRequest(httpRequest: WebResourceLike): Promise<HttpOperationResponse>;
- export declare abstract class FetchHttpClient implements HttpClient {
-     abstract fetch(input: CommonRequestInfo, init?: CommonRequestInit): Promise<CommonResponse>;
-     abstract prepareRequest(httpRequest: WebResourceLike): Promise<Partial<RequestInit>>;
-     abstract processRequest(operationResponse: HttpOperationResponse): Promise<void>;
-     sendRequest(httpRequest: WebResourceLike): Promise<HttpOperationResponse>;
- }

[jeremymeng]

Looks good!

Just for the history the abstraction is because we used to have a browserFetchHttpClient (but never use it and removed it at some point)
https://github.com/Azure/ms-rest-js/blob/master/lib/browserFetchHttpClient.ts

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