adding Meta impl

[jondot]

Meta implementation (see https://github.com/SpectralOps/teller/issues/65)

• [x] add types + empty impls for all providers
• [x] add teller providers export command that outputs all providers, and their meta in JSON. Add teller-specific metadata to this JSON (what version of teller generated it, will be useful for docs)

{ 
     "teller":{
         "version": ".."
      },
      "providers":{ 
          "heroku":{
                 ..
          }
      }
}

• while exporting, if duplicate provider name found -- warn (this is a way for us to start linting providers)

• [ ] add concrete implementation for every provider for their meta
• [ ] replace config template with the relevant provider meta (this means template is being built real-time and/or different templating generation flow)
• [ ] build a tool to generate README automatically from export JSON, add to workflow as a make command
• [ ] teller-docs build a tool to create markdown pages for docs per provider with a standard layout, from exported JSON

[eladbash]

https://github.com/tellerops/teller/issues/125 will be implemented on this branch as well