Allow Agent pods to have Affinity

[juliogreff]

What does this PR do?

It is already possible to configure affinity for cluster-agent and worker deployment but not for the agent. This PR introduces that ability.

Describe your test plan

Create a DatadogAgent CR with spec.agent.affinity set to something (below is an example). Verify that the relevant pods have the same affinity rules.

podAntiAffinity:
  requiredDuringSchedulingIgnoredDuringExecution:
  - labelSelector:
      matchLabels:
        app: datadog-agent
    topologyKey: kubernetes.io/hostname

[codecov-commenter]

Codecov Report

Merging #319 (9a9c965) into main (8072ba9) will increase coverage by 22.86%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##             main     #319       +/-   ##
===========================================
+ Coverage   35.55%   58.42%   +22.86%     
===========================================
  Files          62       60        -2     
  Lines       10667     6362     -4305     
===========================================
- Hits         3793     3717       -76     
+ Misses       6592     2370     -4222     
+ Partials      282      275        -7     

Flag	Coverage Δ
unittests	58.42% <100.00%> (+22.86%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
api/v1alpha1/datadogagent_types.go	100.00% <ø> (ø)
controllers/datadogagent/utils.go	81.57% <100.00%> (+0.01%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8072ba9...9a9c965. Read the comment docs.