Add setRetryStrategy to ApacheHttpClient5TransportBuilder

[loggk]

Description

The builder unconditionally calls disableAutomaticRetries() on the underlying HttpAsyncClientBuilder. In HttpClient 5 this flag cannot be undone, so a retry strategy set through HttpClientConfigCallback is silently ignored and there is no supported way to enable retries.

This adds a setRetryStrategy(HttpRequestRetryStrategy) option to the builder. When set, the strategy is passed through to the http client; when not set, retries stay disabled as before, keeping the default behavior on par with the 4.x http client.

A new RetryStrategyTest verifies both behaviors against a local socket server that fails the first request with a 503: with a retry strategy the request is transparently retried and succeeds, and by default the failure surfaces without a retry.

Issues Resolved

Fixes #1738

• New functionality includes testing
• API changes companion pull request created, if applicable
• Commits are signed per the DCO using --signoff
• Public documentation issue/PR created, if applicable

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