add enum methods

[HassanAkbar]

Glossarist uses enum methods, and multiple values for enum are also supported, so we need to add that to the lutaml model as well.

We can make the values an enum and use collection: true to see if they contain multiple values. For example for the given snippet

class GrammarInfo
  attribute :number, values: %w[singular dual plural], collection: true
end

Current behavior:

The above snippet does not parse multiple values for number i.e `number = %w[dual plural]

Expected behavior:

The above snippet should parse multiple values for number i'e `number = %w[dual plural]

blocking -> glossarist/glossarist-ruby#107