Proposal for long-term changes

# Proposal for long-term changes
There are a number of changes I would like to make that would, in my opinion,
greatly improve the quality, maintainability, and usability of this module.
Some of them are small things, but others are large and would break
compatibility with older versions of the module. I have outlined them below and
am open to input from the maintainers of this module and the greater community.

## Refactoring `init.pp`
The fact that this class does nothing but tell you to use another class does not
make a lot of sense, currently. Historically, this module managed the frontend
and backend components of Graylog. This made a whole lot of sense when those
were actually separate components. When they were merged into one monolithic
service that paradigm made a lot less sense. This change to Graylog made a lot
of sense, but the result for this module was that nearly all of the actual
functionality is accessed through `graylog::server`.

### Proposal
Consolidate the module's main functionality into `init.pp`. This will enable
users to rely on the base class rather than needing the server-specific class
and reflects the current Graylog architecture.

## Class chaining
This whole module requires class chaining to be used in a specific order. This
made a lot of sense with earlier versions of puppet. However, it is pretty
awkward and not a recommended way of developing modules currently.

### Proposal
Include or require sub-classes through `init.pp`. This change will allow users
not to have to worry about the dependency chain of classes. It also completely
eliminates the need for `anchor` resources, the use of which is recommended
against.

## Convert graylog.conf.erb to .epp
Currently, this module uses both `.erb` and `.epp` template formats, which adds
unnecessary complexity to development.

### Proposal
To maintain consistency and simply template management, I propose converting
`graylog.conf.erb` to an `.epp` template.

## Apt repository simplification
The `graylog::repository::apt` class is overly complex due to past limitations
in the `puppetlabs/apt` module. However, over the last years updates to the
`apt` module now allow for a more streamlined approach.

### Proposal
Leveraging the improved functionality of the `apt` module to manage the PGP key,
the repository, and the proxy configuration. This will greatly simplify the
code, improve maintainability, and reduce technical debt.

## Changes to the java environment variables
Currently, this module relies on outdated OS-specific methods to pass
environment variables to the Graylog service which can be inconsistent across
different distributions.

### Proposal
Use systemd overrides to manage environment variables fro the Graylog service.
This approach is simpler, more modern, and compatible with all supported Linux
distributions.

## Remove allinone class
The `graylog::allinone` class does not offer any great functionality, it simply
takes configuration for other modules and passes it to the other modules. The
dependencies for this class are not included in the `metadata.json` file, and
this class does not currently work on RedhHat-based distributions with newer
puppet versions due to the use of legacy facts in the dependent opensearch
module.

### Proposal
Remove this class and replace it with an example. This reduces the dependencies
for this module and simplifies the code base while still providing documentation
for creating an all-in-one system.

## Adding examples in examples directory
This module lacks an examples directory with examples for using this module.

### Proposal
Create some simple examples for using this module to manage a Graylog server.
Doing so will bring this module closer to being able to be submitted for
"Approved" status with Puppet.