Make sure Maven `settings.xml` are read correctly

Maven authentication information is stored in local `settings.xml`.

Considering the extractor uses virtual filesystem, we need to refactor `MavenRegistryAPIClient` to use the virtual filesystem to read the local settings.

`settings.xml` is [read](https://github.com/google/osv-scalibr/blob/main/clients/datasource/maven_registry.go#L81) when we construct `NewMavenRegistryAPIClient` and the client is constructed when the extractor is constructed which is before `Extract()` is called.

Start in clients/datasource/maven_registry.go, especially NewMavenRegistryAPIClient and the settings.xml read at construction time. Trace how the extractor's virtual filesystem becomes available by Extract(); done means Maven authentication settings are read through that filesystem at the appropriate point, with relevant existing tests updated or added.

Written by the indexing model from the issue text.