Registration mechanism instead of symlink based for custom loaders, parsers etc.

[zgornel]

A new mechanism based on explicit registering of loaders, parsers etc. should exist. This allows for

• removal of the custom folders + symlinks directly specifying a custom hook file that includes all custom code (loaders, etc.);The path to the custom hook file should be a gars option.
• This gives the ability for parsers calling to call other parsers, and so on. (i.e. an auto or pre-parser Implicit parser specification #28 )

Mockup

• gars starts with gars -d config.jl --hook ./custom_hooks.jl -p 9000 --log-level debug
• hook file, custom_hooks.jl contains a loader and a parser

@register_loader fload "A nice loader name"
function fload(path) 
    #...
end

@register_parser fparse "An even nicer parser name"
fparse(path)
    # ...
end

Now, a preparser would parse a raw query, extract the parser from a "parser>query" pattern
and execute it (this option could be a default for missing input_parser in requests, with a fallback to noop if no parser can be selected.