Add the configure option for retries, which is easier to understand

[StoneDot]

As discussed in #196 (comment), the current retry configuration can be improved to be easily understood and written. The following is the current configuration.

retry:
  default:
    initial_backoff:
      secs: 0
      nanos: 500000000

We can improve it using enum with untagged. For instance, the following is the potential acceptable configuration.

retry:
  default:
    initial_backoff: 0.5 # This means 500 ms

retry:
  default:
    initial_backoff: 0.1s # This means 500 ms

retry:
  default:
    initial_backoff: 100ms