fix: add context to notification

[saumeya]

What type of PR is this?

/kind bug

What does this PR do / why we need it:
To allpow users to set context in their notificationConfiguration CR

Have you updated the necessary documentation?

• Documentation update is required by this PR.
• Documentation has been updated.

Which issue(s) this PR fixes:

Fixes #?

How to test changes / Special notes to the reviewer:

1. Run the operator locally using make install run
2. Create argo-cd instance with notifications enabled

apiVersion: argoproj.io/v1alpha1
kind: ArgoCD
metadata:
  name: argocd
spec:
  notifications:
    enabled: true

3. Verify the the notification CR default-notifications-configuration is created
4. Add a context to the notification CR and save the CR. By default there is no context set in the CR.

spec:
  context: 
    key1: value1
    key2: value2
  templates:
    template.app-created: |-
      email:
        subject: Application {{.app.metadata.name}} has been created.
      message: Application {{.app.metadata.name}} has been created.
      teams:
        title: Application {{.app.metadata.name}} has been created.
    template.app-deleted: |-
     ...
     ...

5. Check the configmap argocd-notifications-cm , verify the context appears correctly.

[svghadi]

Tested out the changes. Looks good.
Thanks @saumeya !!

[svghadi]

/cherry-pick release-0.10