Provide the ability to fetch content and site via Guillotine Util

[anatol-sialitski]

Provide the ability to fetch content and site via Guillotine Util in the Guillotine extensions.

[rymsha]

After we tried to implement it we found out that there are better alternatives

• best would be a DataFetcher. But we probably need to wait for better async support (GraalJS)
• if/when guillotine needs morefileds lib_content.get provides (_project, _branch) we can do it directly on Content type. So, currently guillotine extension should use lib_content