Create dedicated types for properties of configuration that have been checked

A structure such as

type WebhookConfig struct {
	NotifierConfig `yaml:",inline" json:",inline"`

	HTTPConfig *commoncfg.HTTPClientConfig `yaml:"http_config,omitempty" json:"http_config,omitempty"`

	// URL to send POST request to.
	URL     SecretTemplateURL `yaml:"url,omitempty" json:"url,omitempty"`
	URLFile string            `yaml:"url_file" json:"url_file"`

	// MaxAlerts is the maximum number of alerts to be sent per webhook message.
	// Alerts exceeding this threshold will be truncated. Setting this to 0
	// allows an unlimited number of alerts.
	MaxAlerts uint64 `yaml:"max_alerts" json:"max_alerts"`

	// Timeout is the maximum time allowed to invoke the webhook. Setting this to 0
	// does not impose a timeout.
	Timeout time.Duration `yaml:"timeout" json:"timeout"`
}

is currently exposed. However, this means things such as

 c.URL == "" && c.URLFile == "" {
		return errors.New("one of url or url_file must be configured")
	}

need to be checked again later in the code, because the fields are validated instead of only creating a single field for later use.

The proposed fix here is to have an effective configuration struct, which contains information consumed by the notifiers.

A more in-depth explanation of this idea can be found here Parse, don't validate

Start by locating WebhookConfig and the validation of URL and URLFile, then trace how notifier configuration is consumed. Define the effective configuration boundary described in the issue so notifiers consume checked values without repeating validation; done means the proposed structure is used consistently by the relevant notifier paths.