[ENHANCE] support setting slack channel

[KyriosGN0]

Is your feature request related to a problem? Please describe.
I would like to have a single slack app (which exists in multiple channels) that will serve multiple reloaders by setting which channel to send alerts per reloader instance

Describe the solution you'd like
I would like to set a parameter that will decide to which channel to send the message

Describe alternatives you've considered
create multiple slack app per reloader

Additional context
Add any other context or screenshots about the feature request here.

[mahmadmujtaba]

Hey @KyriosGN0 Thanks for good idea. We are open to any positive contributions and will gladly accept the changes.

[josegonzalez]

I was thinking about this as I would like to notify specific channels whenever an action is taken for a given app/service. This would ensure developers could monitor just the things pertinent to them vs having a stream of notifications for applications they don't care about.

It might be nice if we could have an annotation like reloader.stakater.com/webhook-url. This could be specified on the resource or on the pod, and extracted around here. Prefer the pod unless missing, then grab the resource value.

Important

It must be a webhook url as new slack apps don't allow channel overrides.

The SendWebhookAlert() function would need be modified to take a SendWebhookAlertInput struct, with Message and WebhookURL fields. We would also modify it to prefer the incoming WebhookURL (assigned with the annotation value) if available.

The other alert fields currently specified in values files could also be exposed as annotations, and I'd be happy to implement all of this if that is desirable.

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Alternatively, because of the potential sensitivity of slack channel urls, we could expose a map of available webhook urls? Something like the following:

# a base64-encoded json object, where the key is a lookup and the value is a webhook url
base64_encoded_value="$(echo '{"url-1":"http://example.com","url-2":"http://example.org"}' | base64)"
# -> eyJ1cmwtMSI6Imh0dHA6Ly9leGFtcGxlLmNvbSIsInVybC0yIjoiaHR0cDovL2V4YW1wbGUub3JnIn0K

# this would be in the `reloader.deployment.env.secret`
ALERT_WEBHOOK_URLS="$base64_encoded_value"

Then we'd have an annotation like:

reloader.stakater.com/webhook-url-key: url-1

We would get the lookup key, attempt to find it in the decoded ALERT_WEBHOOK_URLS map, and prefer that if available. If a lookup key is specified but not available, log a warning and use any configured fallback value.