Cluster Agent Auth Lifecycle Improvment

[CharlyF]

What does this PR do?

At the moment, if a token is specified to authenticate the communication between the Datadog Agents and the Datadog Cluster Agent resources will not be rotated if it is changed. This can result in communication errors following up on the upadte of the token.
Secondly, this also adds supports for the conversion of the token when migrating from v1alpha1 to v2alpha1.

This uses the MD5 hashing pattern introduced for Lifecycle improvement.

Motivation

Ensuring smooth migration.

Additional Notes

Anything else we should know when reviewing?

Describe your test plan

Deploying:

apiVersion: datadoghq.com/v2alpha1
kind: DatadogAgent
metadata:
  name: datadog
  namespace: default
spec:
  global:
    clusterAgentToken: abc123 # Make sure it's 32 characters

should result in the creation of the secret with the config hash for the token:

apiVersion: v1
data:
  token: XXXXXXX
kind: Secret
metadata:
  annotations:
    checksum/default-custom-config: 0fe60b5ffa7bdd8c0b26ca3ba0775ff7
  labels:
    app.kubernetes.io/instance: datadog
    app.kubernetes.io/managed-by: datadog-operator
  name: datadog-token
  namespace: default
  ownerReferences:
  - apiVersion: datadoghq.com/v2alpha1
[...]

And both the Datadog Agent and Datadog Cluster Agent pods will have the hash to, ensuring the update if the token is rotated:

➜  ~ k describe pod datadog-cluster-agent-7bf844fcd5-gwtmm
Name:             datadog-cluster-agent-7bf844fcd5-gwtmm
[...]
Labels:           agent.datadoghq.com/component=cluster-agent
[...]
Annotations:      checksum/default-custom-config: 0fe60b5ffa7bdd8c0b26ca3ba0775ff7

(note that this still works if you don't specify the token)

[codecov-commenter]

Codecov Report

Merging #740 (5f6c8c3) into main (1bd631c) will increase coverage by 0.02%.
The diff coverage is 84.21%.

@@            Coverage Diff             @@
##             main     #740      +/-   ##
==========================================
+ Coverage   58.83%   58.85%   +0.02%     
==========================================
  Files         149      149              
  Lines       17846    17863      +17     
==========================================
+ Hits        10499    10513      +14     
- Misses       6721     6723       +2     
- Partials      626      627       +1     

Flag	Coverage Δ
unittests	58.85% <84.21%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
controllers/datadogagent/merger/secret.go	60.71% <72.72%> (+7.77%)	⬆️
apis/datadoghq/v1alpha1/datadogagent_conversion.go	81.15% <100.00%> (+0.30%)	⬆️
...rs/datadogagent/feature/externalmetrics/feature.go	41.07% <100.00%> (+0.60%)	⬆️

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[clamoriniere]

small comments to improve a bit the readability of the code.