Allow credential to be cached using a credential_process

**Is your feature request related to a problem? Please describe.**
People who use my credential plugin want caching of their STS creds. They get throttle by the upstream service vending the creds.

**Describe the solution you'd like**
When using a profile that uses a credential_process, I'd like to be able to add a configuration option to the config to cache the credentials returned by the process, much like what is described here [1] for `AssumeRole` operations.

**Describe alternatives you've considered**
While I _could_ add caching to my application, it seems like exposing this functionality in the AWS CLI makes more sense due to other people needing to utilize caching.

**Additional context**
example ~/.aws/config file

```
[profile prod-us-west-1]
region = us-west-1
credential_process = /path/to/my/process
cache_credentials = true
```

[1] https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-files.html