Add attr_reader options for Puma::Server

[icbd]

Description

Add attr_reader :options for Puma::Server.

On the one hand, it can reduce the use of instance variables.
Using the reader method can more clearly indicate that this is a read-only attribution and will not be changed in other methods.

On the other hand, It allows Puma::Server to expose @options for external use.
I have a scenario that requires this feature: Fetch PUMA Workers count of options in Rails runtime.

When using PUMA Cli to start the service, we can indeed do this ::Puma.cli_config.options[:workers],
but if we start the server through rails server, Puma can not response cli_config method.

# This is a compatible method
Puma::Server.current.instance_variable_get('@options').fetch(:workers)

# hope to use
# Puma::Server.current.options.fetch(:workers)

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[nateberkopec]

I don't see any problem with this, as options are basically a public API already anyway.