Expose RetryClient externally

[erplsf]

Greetings,

This is a bit far-reaching PR. We're using https://github.com/grafana/terraform-provider-grafana which uses this library for Grafana OnCall API.

1. terraform-provider-grafana has an option called insecure_skip_verify which allows to use the provider when hosting Grafana on self-signed (/self-hosted) certificates.
2. Unfortunately, I've recently discovered that the insecure_skip_verify option that is defined, used in parseTLSConfig method which is used only once for configuring main Grafana API client, which in turns mean that we cannot use the provider to connect to Grafana OnCall API with self-signed certificates.

As terraform-provider-grafana uses this library, the easy way to change that is to expose RetryableHTTP client field publicly, so users of this library (terraform-provider-grafana) can use the same parseTLSConfig and pass the appropriate TLS settings to underlying Go HTTP Client.

PR for terraform-provider-grafana will come after this change is merged. Here are the proposed changes (plus the dependency file changes for updating ref to this library) for terraform-provider-grafana: grafana/terraform-provider-grafana@main...klar-mx:terraform-provider-grafana:pr/upstream-changes

[CLAassistant]

All committers have signed the CLA.