Retry Multiplier and Jitter should be float or string

[jjrodenburg]

What happened:
Retry multipliers and Jitters can either be of type float or ISO 8601 duration string per the specification. The code in retry.go currently has the type as intstr.IntOrString. To use a multiplier or jitter as float, you have to parse it from the string. This works, but it seems odd that the SDK allows them to be provided as integers when the spec says they have to be floats or strings.

What you expected to happen:
Be able to provide a float in a non-string representation and get it without having to parse a string. Eg, an equivelant functionality to intstr.IntOrString along the lines of FloatOrString.

How to reproduce it:

Anything else we need to know?:

Environment:

• Specification version used:

• Go version: