Feat: Add Exponential Retry Mechanism with Idempotency Headers

[Eazybright]

This PR added the following features to the SDK:

1. Exponential Retry Mechanism: Failed requests can be retried with configurable paramters

   client = Novu::Client.new(
   access_token: '<your-novu-api_key>', 
   idempotency_key: '<your-idempotency-key>',
   enable_retry: true, # it is disabled by default,
   retry_config: {
   max_retries: 3, 
   initial_delay: 4, 
   max_delay: 60
   }
   )

2. Idempotency Key Provisioning:

   client = Novu::Client.new(
   access_token: '<your-novu-api_key>', 
   idempotency_key: '<your-idempotency-key>'
   )

3. Adds Named Argument to client initialization: Before:

   client = Novu::Client.new('<your-novu-api_key>')

Now:

client = Novu::Client.new(access_token: '<your-novu-api_key>')