Add variable substitution into webhook url parameters

[oreillymj]

Overview

It would be really useful to be able to dynamically put values from an event triggering a webhook into the URL parameters.
For example, it would be nice to expose variables based on JSON path

botkube/kubernetes:
context: *default-plugin-context
enabled: true
config:
namespaces: ns-test
event:
types:
- create
- update
- delete
- error
labels: { "name" : "value" }
updateSetting:
includeDiff: true
fields:
- status.renewalTime
resources:
- type: cert-manager.io/v1/certificates

Then in a webhook url, refer to the values above as follows

webhook:
  enabled: true
  url: "http://echo.cluster.local/param?action=someaction&namespace={config.namespaces.value}&event={config.event.types.value}&label={config.labels.value[0]}&labels={config.label.values}&resource={config.resources.type.value}"

Where multiple items can trigger an event, like create,update then the event which triggered the webhook is available as a value.
Where items like annotations or labels are an array, you can specify the index or pass the entire array as a base64 encoded JSON array.

It would also be nice to refer to fields in the resource, so for example secrets in a certificate can put into the URL

Acceptance Criteria

• Make sure documentation is up-to-date, create one if needed
• Add test coverage for happy-path (unit or e2e test)
• Record raw demo video for the user-facing changes

Reason

It makes the webhook much more useful for passing values into a REST API.