Add credentialProvider as an optional parameter of ConfigurationOptions

As discussed in #1338 the documentation for AWS.S3 and AWS.SES both
state that they accept credentialProvider, which is an instance of
AWS.CredentialProviderChain, as part of the configuration options.

However, the declaration for ConfigurationOptions did not expose such a
parameter. This patch resolves that issue.

Fixes #1338

lib/config.d.ts

@@ -2,6 +2,7 @@ import {Agent as httpAgent} from 'http';
 import {Agent as httpsAgent} from 'https';
 import {AWSError} from './error';
 import {Credentials, CredentialsOptions} from './credentials';
+import {CredentialProviderChain} from './credentials/credential_provider_chain';
 import {ConfigurationServicePlaceholders, ConfigurationServiceApiVersions} from './config_service_placeholders';
 
 export class ConfigBase extends ConfigurationOptions{
@@ -171,6 +172,10 @@ export abstract class ConfigurationOptions {
      * The AWS credentials to sign requests with.
      */
     credentials?: Credentials|CredentialsOptions
+    /**
+     * The provider chain used to resolve credentials if no static credentials property is set.
+     */
+    credentialProvider?: CredentialProviderChain
     /**
      * AWS access key ID.
      * 

ts/config.ts

@@ -93,4 +93,13 @@ var config = AWS.config.loadFromPath('/to/path');
 // test update allowing unknown keys
 AWS.config.update({
    fake: 'fake' 
-}, true);
+}, true);
+
+// Test constructing with a CredentialProviderChain
+var options = {
+    credentialProvider: new AWS.CredentialProviderChain([
+        () => new AWS.EC2MetadataCredentials()
+    ])
+};
+var s3 = new AWS.S3(options);
+var ses = new AWS.SES(options);