[DynamoDB][#1380] Base URL should be overridable in DynamoClientImpl

[tPl0ch]

It is currently impossible to configure the plain HTTP protocol, since the AwsClient trait hard-codes the HTTP protocol scheme.

We need to expose this to the implementations of the AwsClient trait, and in this specific case, the DynamoClientImpl.

• Did a bit of boy-scouting in AwsClient
• Converted the private var url to protected def url: String
• Overrode the url definition in DynamoClientImpl

This PR closes #1380

[tPl0ch]

On a second thought though I realized that this might break downstream clients who have set tls to false and are still requesting the HTTPS endpoints.

Releasing this change would mean that all these clients would then start requesting the unencrypted HTTP endpoints.

Should this be reflected in the documentation?

[ennru]

LGTM.

[ennru]

Great, thank you.