Adding CredentialsProvider, deprecating PasswordProvider

### Motivation

We are using `PasswordProvider` to secure user passwords in various places. However, it doesn't secure the user name (or account whatever).

### Proposed changes

This proposal is to add a new class `CredentialsProvider`. Similar to `PasswordProvider`, the `CredentialsProvider` provides various ways to get the user credential, but both user name and password.

```java
public interface CredentialsProvider
{
Map getCredentials();
}
```

### Rationale

Since the `PasswordProvider` has one method of `getPassword()`, we are already using two different `PasswordProvider`s to secure both user name and password in some places. `AWSCredentialsConfig` is an example:

```java
public class AWSCredentialsConfig
{
@JsonProperty
private PasswordProvider accessKey = new DefaultPasswordProvider("");

@JsonProperty
private PasswordProvider secretKey = new DefaultPasswordProvider("");
...
```

This is not good for user experience because they generally prefer to secure their account and password in the same way.

### Operational impact

The `PasswordProvider` will be deprecated in favor of the new `CredentialsProvider`. This would deprecate some configurations as well, but they will be still supported in a couple of next releases.

Start by locating PasswordProvider, DefaultPasswordProvider, and AWSCredentialsConfig, then review the existing credential-related configurations and their tests. Done means adding the proposed CredentialsProvider interface, supporting both user name and password retrieval, and deprecating PasswordProvider and affected configurations while preserving their stated transitional support.

Written by the indexing model from the issue text.