Making machine identity a parameter of the agent config

[chkeita]

Summary of the Pull Request

This is part of the unmanaged node work

This PR introduces MachineIdentity as new configuration field to the agent and the tasks.
It contains the machine_name, machine_id and scaleset name.
When setting this field on the agent, we bypass the current behavior of reading those values from files or the environment.
This allows multiple instances of the agent on the same machine to behave as different nodes.

The existing behavior stays the same when the MachineIdentity is not specified in the config

closes #2521

[codecov-commenter]

Codecov Report

Merging #2649 (9d1dbb3) into main (ae827b2) will decrease coverage by 4.45%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2649      +/-   ##
==========================================
- Coverage   29.37%   24.91%   -4.46%     
==========================================
  Files         290      121     -169     
  Lines       35603    12536   -23067     
==========================================
- Hits        10457     3123    -7334     
+ Misses      25146     9413   -15733     

Impacted Files	Coverage Δ
src/agent/onefuzz-agent/src/agent.rs
src/agent/onefuzz-agent/src/commands.rs
src/agent/onefuzz-agent/src/config.rs
src/agent/onefuzz-agent/src/heartbeat.rs
src/agent/onefuzz-agent/src/main.rs
src/agent/onefuzz-agent/src/scheduler.rs
src/agent/onefuzz-agent/src/worker.rs
...ent/onefuzz-task/src/local/libfuzzer_test_input.rs
src/agent/onefuzz-task/src/local/test_input.rs
src/agent/onefuzz-task/src/managed/cmd.rs
... and 159 more

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