Cgroupsv2 container

[kanderson250]

Resolves #1358

This PR adds support for getting the containerId from a docker container with Linux cgroup v2.

Previously, the containerId was grabbed from proc/self/cgroup for containers with cgroup V1. Now, the id is grabbed from proc/self/mountinfo, with fallthrough to proc/self/cgroup if that file is missing.

• This is the widely accepted approach for now, though the API is expected to change and a long-term solution is supposedly in the works.
• The current implementation is based on best-effort research towards the contents of proc/self/mountinfo. Because this solution is a bit hacky, there isn't clear documentation about what's supposed to be in there in different environments/providers. The matching logic may need to be updated in the future.

[codecov-commenter]

Codecov Report

Merging #1529 (f968855) into main (2c20813) will decrease coverage by 0.01%.
Report is 8 commits behind head on main.
The diff coverage is 77.77%.

@@             Coverage Diff              @@
##               main    #1529      +/-   ##
============================================
- Coverage     70.60%   70.59%   -0.01%     
- Complexity     9745     9753       +8     
============================================
  Files           813      814       +1     
  Lines         39264    39299      +35     
  Branches       5962     5968       +6     
============================================
+ Hits          27723    27744      +21     
- Misses         8856     8864       +8     
- Partials       2685     2691       +6     

Files	Coverage Δ
...in/java/com/newrelic/agent/utilization/CGroup.java	100.00% <100.00%> (ø)
...ava/com/newrelic/agent/utilization/DockerData.java	80.00% <75.00%> (-4.10%)	⬇️

... and 11 files with indirect coverage changes

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[jtduffy]

2 minor nits. Nice test coverage!