Support append/prepend operations for trigger parameters

[marxarelli]

Is your feature request related to a problem? Please describe.
I'm looking to use Argo Events to respond to patchset-created events generated by Gerrit code review, creating a workflow that runs CI tasks based on a workflow file stored in the patchset's project repo. Gerrit does not include the full clone URL for the project in the event data and so it must be constructed from a prefix and the project name. However, this isn't possible given the fact that templateParameters can only overwrite values, not prepend/append existing ones.

Describe the solution you'd like
I would love a TriggerParameter.Operation field of some sort that supported keywords such as overwrite (default), prepend, and append. (Or something similar that achieves the desired functionality.) I'm down to author a pull request if that's helpful but I thought I'd ask first to judge how it might be received.

Describe alternatives you've considered
I've thought about writing a Gerrit plugin to include additional information in the event payload but then I'd have to write more Java and that's no fun.

Additional context
Full example of what I'm hoping for:

apiVersion: argoproj.io/v1alpha1
kind: Sensor
metadata:
  name: gerrit-webhook-sensor
  labels:
    sensors.argoproj.io/sensor-controller-instanceid: argo-events
    argo-events-sensor-version: v0.10
spec:
  template:
    spec:
      containers:
        - name: "sensor"
          image: "argoproj/sensor"
          imagePullPolicy: Always
      serviceAccountName: argo-events-sa
  dependencies:
    - name: "gerrit-webhook-gateway:gerrit-events"
      filters:
        context:
          # source: { host: gerrit.git.wmflabs.org }
          contentType: application/json
        data:
          - path: type
            type: string
            value: [ patchset-created ]
  eventProtocol:
    type: "HTTP"
    http:
      port: "9300"
  triggers:
    - template:
        name: gerrit-webhook-workflow-trigger
        group: argoproj.io
        version: v1alpha1
        kind: Workflow
        source:
          git:
            url: https://gerrit.git.wmflabs.org/r/
            cloneDirectory: /src
            filePath: .pipeline/argo-workflow.yaml
            branch: REPLACED_WITH_PATCHSET_REF
      templateParameters:
        - src:
            event: "gerrit-webhook-gateway:gerrit-events"
            path: patchSet.ref
          dest: source.git.branch
        - src:
            event: "gerrit-webhook-gateway:gerrit-events"
            path: change.project
          dest: source.git.url
          operation: append # would <3 if i could do this

[VaibhavPage]

Looks like a nice feature to have. I'll add it in the next release.