Add indexName field to AWS DynamoDB scaler

[Haegi]

Proposal

The AWS DynamoDB scaler should support the IndexName argument for dynamo db queries.
At the moment there is not possibility to specify the IndexName in for the AWS DynamoDB scaler.
This argument is quite important when the dynamo query should run using an index.

Proposal:

• Add indexName to AWS DynamoDB scaler (metadata).
• The field should be optional.

Use-Case

Running dynamo db queries using an index is a common use-case. Especially, when specifying multiple keys is not an option and indices are used in the table.

Is this a feature you are interested in implementing yourself?

Yes

Anything else?

No response