Make `min_available_memory_mb` configurable

[kananb]

Summary of the Pull Request

Makes min_available_memory_mb configurable per task from the CLI and service.

PR Checklist

• [x] Applies to work item: #3518
• [ ] CLA signed. If not, go over here and sign the CLI.
• [ ] Tests added/passed
• [ ] Requires documentation to be updated
• [ ] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

Info on Pull Request

What does this include?

Validation Steps Performed

How does someone test & validate?

[codecov-commenter]

Codecov Report

Merging #3577 (ea6217b) into main (679309a) will increase coverage by 0.00%. The diff coverage is 12.50%.

@@           Coverage Diff           @@
##             main    #3577   +/-   ##
=======================================
  Coverage   39.00%   39.00%           
=======================================
  Files         302      302           
  Lines       36958    36963    +5     
=======================================
+ Hits        14414    14419    +5     
  Misses      22544    22544           

Files	Coverage Δ
...e/onefuzzlib/notifications/JinjaTemplateAdapter.cs	90.88% <100.00%> (+0.02%)	:arrow_up:
src/ApiService/ApiService/OneFuzzTypes/Model.cs	70.51% <50.00%> (-0.05%)	:arrow_down:
src/ApiService/ApiService/onefuzzlib/Config.cs	0.00% <0.00%> (ø)
src/agent/onefuzz-task/src/managed/cmd.rs	0.00% <0.00%> (ø)

... and 2 files with indirect coverage changes