feat(lacework-agent): add vmprovidercustom value (#213)

This commit adds the `vmprovidercustom` property to the `lacework-agent`
Helm charts.

Specifically, this commit:

* Adds an entry in the JSON schema for the `vmprovidercustom` key
* Adds templating to the configmap to render the `vmprovidercustom`
  config JSON if the Helm option is enabled
* Updates the `values.yaml` file to document this new Helm value

Resolves LINK-2104