[Feature Request] support for passing custom http client (or axios instance)

[daisy1754]

Is your feature request related to a problem? Please describe.
We want to pass custom http client to sdk so we can instrument request latency etc using interceptor code we have across our code base.

Describe the solution you'd like
Ability to pass http client via cosntructor arg to sdk. For instance here is how stripe-node support custom http client
https://github.com/stripe/stripe-node/blob/master/types/lib.d.ts#L73

[alfonso-paella]

This would allow for the usage of connection pooling, for ex., which would be a very necessary feature for apps handling high throughput applications reliant on Fireblocks.