Support pulling configuration from AWS SSM Parameter Store

**Is your feature request related to a problem? Please describe.**

As described in this comment, users sometimes need to store sensitive information (passwords) in the Fluent Bit config file. In AWS, SSM parameter store or secrets manager is the preferred solution: https://github.com/fluent/fluent-bit/issues/591#issuecomment-590579805

(This feature request is AWS focused, and I work for AWS; non-AWS solutions to this problem are encouraged/appreciated as well).

More generally, storing the entire Fluent Bit configuration in a centralized location is potentially useful. Kubernetes users can configure a config map which is then used as the config for all Fluent Bit daemons. If you're running outside of kubernetes, some other solution is needed.

**Describe the solution you'd like**

For SSM paramater store, you could invoke Fluent Bit with an argument that would tell it to download configuration:

```
$ ./bin/fluent-bit -c arn:aws:ssm:us-west-2:1111111111111:parameter/my-fluent-bit-config
```