Add hook for instance startup

[tantalic]

1. What does this change do, exactly?

Provides a new hook that plugins can use to gain access to the caddy.Instance. This is particularly important to allow plugins the ability to dynamically generate a new Caddyfile and reload the server with the new configuration.

With this change in place a plugin could access the instance as follows:

func init() {
	caddy.RegisterEventHook("sampleinithook", SampleInitHook)
}

func SampleInitHook(eventType caddy.EventName, eventInfo interface{}) error {
	if eventType != caddy.InstanceStartupEvent {
		return nil
	}

	instance, ok := eventInfo.(*caddy.Instance)
	if !ok {
		return errors.New("Invalid eventInfo provided for caddy.InstanceStartupEvent event")
	}

	// save the instance for later use and/or do something useful with it here
}

2. Please link to the relevant issues.

• Wiki documents this as a TODO: https://github.com/mholt/caddy/wiki/Writing-a-Plugin:-Caddyfile-Loader#dynamic-reloads
• Community discussion: https://caddy.community/t/dynamically-load-caddy-file/357/11

3. Which documentation changes (if any) need to be made because of this PR?

• Caddyfile Loader: https://github.com/mholt/caddy/wiki/Writing-a-Plugin:-Caddyfile-Loader#dynamic-reloads

4. Checklist

• I have written tests and verified that they fail without my change - I have verified existing tests pass but it appears the events are not currently included in tests and I see no obvious way to add tests for this case
• I have squashed any insignificant commits
• This change has comments for package types, values, functions, and non-obvious lines of code
• I am willing to help maintain this change if there are issues with it later

[CLAassistant]

All committers have signed the CLA.

[elcore]

Hello @tantalic,

I am going to close this PR as I already implemented it (Reference : #1879)

[elcore]

I will add this to my PR 😄

[mholt]

Hey Eldin, I think it would be fair to make sure that @tantalic gets the credit in the commit; it's OK if there are two separate PRs. Smaller PRs are better. 😉

[elcore]

Hey Eldin, I think it would be fair to make sure that @tantalic gets the credit in the commit; it's OK if there are two separate PRs. Smaller PRs are better. 😉

I have no issues giving credit for the first part of this PR ☺️ - - It looks like this PR is based upon my changes. (Reference : elcore@7056893)

You can merge this PR. I'll update my PR....

[mholt]

@tantalic Can you please rebase with master? I'll begin review after the merge conflict is resolved.

[tantalic]

@mholt - The changes are now rebased on master. It appears CI builds are failing for some reason outside the scope of this change now. Please advise if you need anything else on this PR.

[mholt]

Thanks for contributing! Really appreciate it. This'll go out in the next release.