Fix typings for credentials provider chain (#3510)

`Credentials[]|providers[]` means either an array with `Credentials` or
an array with `providers`. But in reality it's a array with both.

Author: workeitel

.changes/next-release/bugfix-Types-7306bc23.json

@@ -0,0 +1,5 @@
+{
+  "type": "bugfix",
+  "category": "Types",
+  "description": "Update types for credential provider chain"
+}

lib/credentials/credential_provider_chain.d.ts

@@ -8,15 +8,15 @@ export class CredentialProviderChain {
     /**
      * Resolves the provider chain by searching for the first set of credentials in providers.
      */
-    resolve(callback:(err: AWSError, credentials: Credentials) => void): CredentialProviderChain;
+    resolve(callback:(err: AWSError|null, credentials?: Credentials) => void): CredentialProviderChain;
     /**
      * Return a Promise on resolve() function
      */
     resolvePromise(): Promise<Credentials>;
     /**
      * Returns a list of credentials objects or functions that return credentials objects. If the provider is a function, the function will be executed lazily when the provider needs to be checked for valid credentials. By default, this object will be set to the defaultProviders.
      */
-    providers: Credentials[]|provider[];
+    providers: Array<Credentials|provider>;
 
     static defaultProviders: provider[]
 }