Add core agent business logic

[chrisdoherty4]

Summary

This PR adds core business logic for the new tink agent component. The agent will replace the tink worker and can therefore be considered a re-write. Re-writing the worker has allowed us to remove unsupported functionality such as multi-worker workflows that were implemented in the previous Tinkerbell versions as 'tasks'.

The core business logic leverages a transport to retrieve workflows. It communicates workflow progression via events. It runs workflows serially rejecting requests to run concurrently with an explicit event.WorkflowRejected event. It assumes enforcement of state machines will be performed by the server component it interacts with.

Finally, the logging around the agent has been significantly improved to reflect exactly what's happening as it transitions between actions.

Relevant designs/PRs

• v1alpha2 API
• Failure reason and message handling
• Tink agent PoC

Future work

Separate PRs will introduce container runtime and transport implementations. See the Tink agent PoC for a reference.

[codecov]

Codecov Report

Merging #705 (bdb2746) into main (c73414e) will increase coverage by 2.80%.
The diff coverage is 79.82%.

❗ Current head bdb2746 differs from pull request most recent head ae7fc75. Consider uploading reports for the commit ae7fc75 to get more accurate results

@@            Coverage Diff             @@
##             main     #705      +/-   ##
==========================================
+ Coverage   52.73%   55.54%   +2.80%     
==========================================
  Files          23       26       +3     
  Lines         986     1100     +114     
==========================================
+ Hits          520      611      +91     
- Misses        454      474      +20     
- Partials       12       15       +3     

Impacted Files	Coverage Δ
internal/agent/mocks.go	53.33% <53.33%> (ø)
internal/agent/agent.go	92.00% <92.00%> (ø)
internal/agent/run.go	100.00% <100.00%> (ø)

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