[dynamodb] Allow multiple credentials sources and types

[eliransin]

The dynamodb binding was very strict about the credentials, it was mandatory to provide the credentials as a properties files, also, the credentials couldn't be temporary. This change adds two more options on top of the originally supported credentials:

1. It makes the dynamodb.awsCredentialsFile property optional, if this property is not given, the library will search for default credentials according to it's search rules that can be reviewed at: https://docs.aws.amazon.com/sdk-for-java/v1/developer-guide/credentials.html
2. It adds one more supported property to the credentials file schema sessionToken, this adds the ability to generate and use temporary credentials from aws sts command. For more information: https://docs.aws.amazon.com/cli/latest/reference/sts/get-session-token.html#get-session-token

This has been tested against DynamoDB with all 3 methods.

Signed-off-by: Eliran Sinvani eliransin@scylladb.com