Enable connection pooling with UrlTransporter

### New feature, improvement proposal

Currently each connection being established via the UrlTransporter is unconditionally released for example in https://github.com/apache/maven-resolver/blob/45482a82ce0377d0c7a0d8fc343eeff6b94012ad/maven-resolver-transport-url/src/main/java/org/eclipse/aether/transport/url/UrlTransporter.java#L251. Although this is safe in terms of releasing the http connection it would also prevent connection pooling.

Instead one should rather just close the returned input streams. Compare with the Javadoc in https://docs.oracle.com/javase/8/docs/api/java/net/HttpURLConnection.html.

It is important though to close both [regular](https://docs.oracle.com/javase/8/docs/api/java/net/URLConnection.html#getInputStream--) and potentially also [error input stream](https://docs.oracle.com/javase/8/docs/api/java/net/HttpURLConnection.html#getErrorStream--).

Particularly as TLS handshakes are expensive connection pooling usually has a big performance impact.

Start in maven-resolver-transport-url/src/main/java/org/eclipse/aether/transport/url/UrlTransporter.java around line 251, and compare the connection lifecycle with the Java URLConnection and HttpURLConnection Javadocs. Verify how regular and error input streams are obtained and closed. Done means the transporter no longer unconditionally releases each connection, while both possible input streams are closed so connection pooling can work.

Written by the indexing model from the issue text.