@spec fetch(keyword()) :: String.t() | nil

Main logic to extract the token of the bot.

The options can have the following keys:

• token: Explicit token to be used, this ignore everything else and will use this one
• bot: Bot name to be used, the token will be retrieved from the registry
• registry: Optional. Registry to extract the bot's token. By default will use ExGram.Registry.

Usage:

ExGram.Token.fetch() # Will take it from the config (config :ex_gram, token: "token")

ExGram.Token.fetch(token: "MyToken") # Will use the token

ExGram.Token.fetch(bot: :my_bot) # Will look in ExGram.Registry for :my_bot token

ExGram.Token.fetch(bot: :my_bot, registry: OtherRegistry) # Will look in OtherRegistry for :My_bot token