Cache evection of HTTP Client lead to not closed HttpClients

Describe the Bug

The current implementation of caching and evicting an HTTP Client lead to not closed HttpClients.

https://github.com/SAP/cloud-sdk-java/blob/4fb1f6a0a4a2517a10dc4b02c69a6f072d98db8a/cloudplatform/connectivity-apache-httpclient5/src/main/java/com/sap/cloud/sdk/cloudplatform/connectivity/DefaultApacheHttpClient5Cache.java#L42-L47

Technically the HttpClients are CloseableHttpClient and should / need(specifically for pooled connections) to be closed after usage.
(See also this discussion)

In the current code-base, when a HttpClient with a PoolingHttpClientConnectionManager is created, it's not cleaned up properly on eviction.

---

The following workaround does not work in all cases:

        cache = Caffeine.newBuilder().expireAfterAccess(duration, unit).ticker(ticker).evictionListener((key, value, cause) -> {
            if (value instanceof CloseableHttpClient closeableHttpClient) {
                try {
                    closeableHttpClient.close();
                } catch (final Exception e) {
                    log.warn("Failed to close HttpClient. Ignoring the exception and continue.", e);
                }
            }
        }).build();

There are two basic scenarios:

1. The client was created and is no longer in use and the eviction time triggers the cleanup. ✅
2. The client was created and in still in use (long running operation, async operation, ...) and the eviction time triggers the cleanup. Then the evictionListener would kill the connection underneath. ❌

Steps to Reproduce

Code review.

Expected Behavior

Proper closing of HttpClients.

Screenshots

No response

Used Versions

Current state in main.

Code Examples

// Your code here

Stack Trace

No response

Log File

Log file

...

Affected Development Phase

Development

Impact

No Impact

Timeline

No response

从链接的缓存构造处的 DefaultApacheHttpClient5Cache.java 开始，并查看 issue 755 中的讨论。跟踪池化的 CloseableHttpClients 在长时间运行或异步操作期间是如何创建、缓存、驱逐和使用的；完成标准是，被驱逐的客户端能够关闭，同时不中断仍在使用的客户端。