base - functionality for method retry on failure

[ttodua]

    ex.options['retryFailedRequests'] = 1 // default is 0
    ex.options['retryFailedRequestsDelay'] = 100 // default it 0

[ttodua]

System.AggregateException: 'One or more errors occurred. (Unable to cast object of type 'DelayPromise' to type 'System.Threading.Tasks.Task`1[System.Object]'.)'

[ttodua]

@carlosmiei PR passes actually.

[carlosmiei]

two things, we're missing the docs and maybe an example showcasing the feature second, don't we need to update the c# side?

[ttodua]

1) added docs 2) the c# does not need to be updated as the fetch2 method is transpiled (i've neither amended any method in py/php)

failedRequestRetries could have been some possible shorter name too, but as is, i've pushed with maxRetriesOnFailure,