Upgrade SDK and test discovery-ec2 credential providers

[ywelsch]

Upgrades the AWS SDK to the same version that we're using for the repository-s3 plugin, providing testing capabilities to override certain SDK endpoints in order to point them to localhost for testing. Adds tests for the various credential providers.

Relates to #41630 (comment)

[elasticmachine]

Pinging @elastic/es-distributed

[DaveCTurner]

Could we fix the usage of the deprecated new AmazonEC2Client() constructor in AwsEc2ServiceImpl#buildClient() too? This is how it was done in S3Service:

elasticsearch/plugins/repository-s3/src/main/java/org/elasticsearch/repositories/s3/S3Service.java

Lines 139 to 159 in 57823c4

	AmazonS3 buildClient(final S3ClientSettings clientSettings) {
	final AmazonS3ClientBuilder builder = AmazonS3ClientBuilder.standard();
	builder.withCredentials(buildCredentials(logger, clientSettings));
	builder.withClientConfiguration(buildConfiguration(clientSettings));
	final String endpoint = Strings.hasLength(clientSettings.endpoint) ? clientSettings.endpoint : Constants.S3_HOSTNAME;
	logger.debug("using endpoint [{}]", endpoint);
	// If the endpoint configuration isn't set on the builder then the default behaviour is to try
	// and work out what region we are in and use an appropriate endpoint - see AwsClientBuilder#setRegion.
	// In contrast, directly-constructed clients use s3.amazonaws.com unless otherwise instructed. We currently
	// use a directly-constructed client, and need to keep the existing behaviour to avoid a breaking change,
	// so to move to using the builder we must set it explicitly to keep the existing behaviour.
	//
	// We do this because directly constructing the client is deprecated (was already deprecated in 1.1.223 too)
	// so this change removes that usage of a deprecated API.
	builder.withEndpointConfiguration(new AwsClientBuilder.EndpointConfiguration(endpoint, null))
	.enablePathStyleAccess();
	return builder.build();
	}

[ywelsch]

Could we fix the usage of the deprecated new AmazonEC2Client() constructor in AwsEc2ServiceImpl#buildClient() too?

I would rather not in this PR, as that entails other changes (namely change in default endpoint, see #27925)

Requested change is tracked in #27925.

[DaveCTurner]

Ok, LGTM