feat: support namespaced Datadog credentials

[ariadnarouco]

Summary

This pull adds ability to use namespaced credentials for Datadog metrics provider.

Background

Our use case involves an Argo Rollout that operates across multiple namespaces, each managed by tenants with distinct Datadog accounts. As a result, multiple credentials are needed to integrate seamlessly with Datadog.

Implementation

This setup will allow users to use different credentials than the one setup as default, which is a secret named "datadog" deployed in the same namespaces as the rollouts controller [refer to doc].

This way the users can manage their datadog credentials with the name they want and in their own namespace.

The process suggested within this PR for retrieving Datadog credentials is as follows:
1. If a secretRef is defined in the AnalysisTemplate: Argo Rollouts will search for the secret with the specified name in the namespace where the template resides.
2. If the secret is not found in the specified namespace: Argo Rollouts will then check the environment variables.
3. If the credentials are not found in environment variables: Argo Rollouts will look for a secret named "Datadog" in the namespace where Argo Rollouts itself is deployed.

Notes:
Based on the code, to have more than one set of credentials is not possible. I also created a question for this which remains unanswered

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this is a chore.
• The title of the PR is (a) conventional with a list of types and scopes found here, (b) states what changed, and (c) suffixes the related issues number. E.g. "fix(controller): Updates such and such. Fixes #1234".
• I've signed my commits with DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My builds are green. Try syncing with master if they are not.
• My organization is added to USERS.md.

[github-actions]

Published E2E Test Results

  4 files    4 suites   3h 25m 34s ⏱️
111 tests 102 ✅  6 💤 3 ❌
450 runs  420 ✅ 24 💤 6 ❌

For more details on these failures, see this check.

Results for commit 4da856b.

♻️ This comment has been updated with latest results.

[github-actions]

Published Unit Test Results

2 174 tests   2 174 ✅  2m 54s ⏱️
  119 suites      0 💤
    1 files        0 ❌

Results for commit 4da856b.

♻️ This comment has been updated with latest results.