Updated declaration merging interface, made serviceKey optional, exported plugin opts interface

[vehm]

Did a few things:

• Updated the declaration merging FastifyRequest interface to include supabaseClient.
• Made serviceKey optional to allow for users to register the plugin without creating a potentially dangerous client (using service_role) that bypasses all RLS policies for Supabase.
• Exported the FastifySupabasePluginOpts from index.ts

Fixes #28
Fixes #34

[vehm]

Updated Fastify decoration reference type for v5 support:
Decorating Request/Reply with a reference type (Array, Object) is now prohibited as this reference is shared amongst all requests.