Upgrade 1.1 to 2.0

TL-DR

Plugin structure upgraded to PluginSkeleton:^1.3
Removed the all.xml endpoint - use the sitemap index
Sitemaps are now generated via the command line, see below.
Dropped support for relative URL's
Models (& their interfaces) renamed
Drop suggestion that other formats than XML were supported

New features

Generation of sitemaps is done via the CLI, schedule them in a cronjob:
- Sitemap Index: bin/console sylius:sitemap:generate-index
Sitemap URLs now support adding images. The default providers do this where possible. It can be disabled using the images configuration key.

Removed features

Dropped support for relative URL's; Google advises to use fully qualified URL's.
Unintentionally the plugin could suggest that other formats than XML were allowed. This was never properly supported and therefore removed.
Removed the all.xml endpoint, which put all URL's in a single file. It's better to use the index file.

Config changes

Config file extensions changed from yml to yaml

Class changes

Several classes have been marked final.
Models (& their interface) were renamed. Basically 'Sitemap' was removed from the names where relevant (i.e. where the model is not a sitemap, but part of a sitemap)

Interface changes

Interface UrlInterface has new methods:
- getImages(): Collection
- setImages(Collection $images): void
- addImage(SitemapImageUrlInterface $image): void
- hasImage(SitemapImageUrlInterface $image): bool
- removeImage(SitemapImageUrlInterface $image): void
- public function hasImages(): bool
Providers now need to have a ChannelContext supplied.