Standardise CredentialsProvider config between KCL and KPL

[markglh]

Currently the KPL requires you to pass it as an argument, whereas the KCL requires it as config.

• Ideally we should be able to specify both as config
• Verify that custom providers work for both the KCL and KPL

The KCL uses a different (more sophisticated) method for parsing the properties compared to the KPL, see below for an example of how it parses the CredentialsProvider:
https://github.com/awslabs/amazon-kinesis-client/blob/master/src/main/java/com/amazonaws/services/kinesis/clientlibrary/config/KinesisClientLibConfigurator.java#L112

The KPL only supports setting of Strings, Ints and Booleans:
https://github.com/awslabs/amazon-kinesis-producer/blob/master/java/amazon-kinesis-producer/src/main/java/com/amazonaws/services/kinesis/producer/KinesisProducerConfiguration.java#L197