Selenium Grid: Add trigger param to set custom capabilities for matching specific Nodes

[VietND96]

Provide a description of what has been changed

Besides current main capabilities browserName, browserVersion, platformName are used for matching Node. Selenium Grid supports setting more custom capabilities for matching specific Nodes https://www.selenium.dev/documentation/grid/configuration/toml_options/#setting-custom-capabilities-for-matching-specific-nodes
Via this PR, one more trigger parameter is added with the name capabilities to define a JSON string with custom capabilities for matching specific Nodes. This helps users create more scalers to scale their Nodes with custom capabilities.

For example

kind: Deployment
metadata:
  name: selenium-node-chrome
  labels:
    deploymentName: selenium-node-chrome
spec:
  replicas: 1
  template:
    spec:
      containers:
        - name: selenium-node-chrome
          image: selenium/node-chrome:132.0
          ports:
            - containerPort: 5555
          env:
            - name: SE_NODE_BROWSER_VERSION
              value: '132.0'
            - name: SE_NODE_PLATFORM_NAME
              value: 'Linux'
            # Append custom capabilities to Node stereotype. See: https://github.com/SeleniumHQ/docker-selenium?tab=readme-ov-file#node-configuration-options
            - name: SE_NODE_STEREOTYPE_EXTRA
              value: "{\"myApp:version\":\"beta\", \"myApp:publish:\":\"public\"}"
---
apiVersion: keda.sh/v1alpha1
kind: ScaledObject
metadata:
  name: selenium-grid-scaledobject-chrome-132
  namespace: keda
  labels:
    deploymentName: selenium-node-chrome-132
spec:
  maxReplicaCount: 8
  scaleTargetRef:
    name: selenium-node-chrome-132
  triggers:
    - type: selenium-grid
      metadata:
        url: 'http://selenium-hub:4444/graphql'
        browserName: 'chrome'
        platformName: 'Linux'
        browserVersion: '132.0'
        unsafeSsl: 'true'
        # Add custom capabilities for matching specific Nodes in scaler trigger metadata. See: https://github.com/kedacore/keda/pull/6536
        capabilities: "{\"myApp:version\":\"beta\", \"myApp:publish:\":\"public\"}"

The request to trigger this scaler should be

options = ChromeOptions()
options.set_capability('platformName', 'Linux')
options.set_capability('browserVersion', '132.0')
# Add custom capabilities for matching specific Nodes in client binding. See: https://www.selenium.dev/documentation/grid/configuration/toml_options/#setting-custom-capabilities-for-matching-specific-nodes
options.set_capability('myApp:version', 'beta')
options.set_capability('myApp:publish', 'public')
driver = webdriver.Remote(options=options, command_executor=SELENIUM_GRID_URL)

Checklist

• When introducing a new scaler, I agree with the scaling governance policy
• I have verified that my change is according to the deprecations & breaking changes policy
• Tests have been added
• Changelog has been updated and is aligned with our changelog requirements
• A PR is opened to update the documentation on (repo) (if applicable)
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

Fixes #

Relates to #

[zroubalik]

/run-e2e selenium
Update: You can check the progress here