* Update all files using Mini license header to use full ASF header

* fix broken link in dockerfile to released golang TAR

* fix broken link in dockerfile to released golang TAR

Using `object`s instead of a hollow class as the "factory" for an Spi makes them being singleton by default, which reduces boilerplate to implement an Spi.

The `Dependencies` object is not used anywhere and there might be better alternatives which we can decide on once the need of them comes up. For now, the Spi's interfaces are relatively tightly coupled to their current default implementations anyway.

Also updated documentation.

Adds the ability to add pluggable implementations for defined Service Provider Interfaces (SPI). The implementation to load is chosen via configuration.

First set of plug-points are:
- ArtifactStoreProvider
- MessagingProvider