Make container commands more suitable for use as a library

[Furisto]

The idea behind this PR is to make it easier to use youki as library, so it can be reused in different contexts.

let mut container = ContainerBuilder::new(self.container_id.clone(), syscall.as_ref())
            .build()?;
container.start()?;
container.pause()?;
container.resume()?;
container.kill()?;
container.delete()?;

[utam0k]

@alfonsoros88 Hi! Maybe you are interested in this PR. so I'll give you the mention.

[codecov-commenter]

Codecov Report

Merging #314 (a4bac01) into main (2af5216) will decrease coverage by 2.14%.
The diff coverage is 14.75%.

@@            Coverage Diff             @@
##             main     #314      +/-   ##
==========================================
- Coverage   73.09%   70.94%   -2.15%     
==========================================
  Files          44       46       +2     
  Lines        6541     6811     +270     
==========================================
+ Hits         4781     4832      +51     
- Misses       1760     1979     +219     

[Furisto]

• I also have found that some commands used the systemd global option to determine if systemd should be used. Instead the decision has to be made by examining the container state as once the container has been created we have to use the same cgroup manager throughout the lifecycle of the container.
• Added context to many more results.

[utam0k]

perfect!