[MRESOLVER-126] Support property type conversion from String into Map

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[MRESOLVER-126](https://issues.apache.org/jira/browse/MRESOLVER-126?redirect=false)** and commented

The property `aether.connector.http.headers` requires a `Map` as value (https://github.com/apache/maven-resolver/blob/47edcfe69c4e52ced4cb93d65b7348b5645cdd68/maven-resolver-transport-http/src/main/java/org/eclipse/aether/transport/http/HttpTransporter.java#L139). As in https://github.com/apache/maven-resolver/blob/47edcfe69c4e52ced4cb93d65b7348b5645cdd68/maven-resolver-util/src/main/java/org/eclipse/aether/util/ConfigUtils.java#L362 there is no type conversion, it is currently impossible to configure this from Apache Maven (as Maven is using system properties).

`List` suffers from the same issue but is currently not used (https://github.com/apache/maven-resolver/blob/47edcfe69c4e52ced4cb93d65b7348b5645cdd68/maven-resolver-util/src/main/java/org/eclipse/aether/util/ConfigUtils.java#L319)

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**Remote Links:**
- [GitHub Pull Request #63
](https://github.com/apache/maven-resolver/pull/63)

Start in maven-resolver-util/src/main/java/org/eclipse/aether/util/ConfigUtils.java around lines 319 and 362, then inspect maven-resolver-transport-http/src/main/java/org/eclipse/aether/transport/http/HttpTransporter.java around line 139. Review pull request #63 and the existing tests; done means system properties can configure the Map value, with List handling addressed as described.

Written by the indexing model from the issue text.