Fetch config from AWS S3.

Expects the environment variable KUROSAWA_S3_CONFIG_URI to be set to something
like:

s3://my-bucket/path/to/the/things

Unencrypted JSON files located at
- s3://my-bucket/path/to/the/things/configs/config-1.json 
- s3://my-bucket/path/to/the/things/configs/config-2.json 

and KMS encrypted JSON files located at
- s3://my-bucket/path/to/the/things/secrets/config-2.json
- s3://my-bucket/path/to/the/things/secrets/config-3.json

will be read (merged) into a map like

{config-1 {key1 value1
           key2 value2}
 config-2 {key3 value3
           key4 value4}
 config-3 {key3 value3
           key4 value4}}

Fetch config from the SSM parameter store.