feat: add environments module

[Joao-Moura]

• Adds Novu.Environments module with:
  • get_current_environment/0
  • create_environment/2
  • get_environments/0
  • update_environment/2
  • get_api_keys/0
  • regenerate_api_keys/0
• Add support to post requests with empty body using post/2

[Cliftonz]

@doomspork Just to make you aware of another PR review.

[doomspork]

I'm aware @Cliftonz 😁 There were changes just moments ago 👍