Read Proxy Password from Credential Providers in S3 FileSystem

HADOOP-12548 added credential provider support for the AWS credentials to S3FileSystem. This JIRA is for considering the use of the credential providers for the proxy password as well.

Instead of adding the proxy password to the config file directly and in clear text, we could provision it in addition to the AWS credentials into a credential provider and keep it out of clear text.

In terms of usage, it could be added to the same credential store as the AWS credentials or potentially to a more universally available path - since it is the same for everyone. This would however require multiple providers to be configured in the provider.path property and more open file permissions on the store itself.