feat(schemaregistry): allow custom http client

[liveFreeOrCode]

This patch introduces the ability to set a custom base http client that will be used for the rest calls to the Schema Registry.

Common use cases include using traced http clients from telemetry providers. (This may be a viable solution for #712)

// example
package main

import (
	"net/http"

	"github.com/confluentinc/confluent-kafka-go/schemaregistry"
	httptrace "gopkg.in/DataDog/dd-trace-go.v1/contrib/net/http"
)

func main() {
	conf := schemaregistry.NewConfig()
	conf.BaseClient = httptrace.WrapClient(new(http.Client))
	client, err := schemaregistry.NewClient(conf)
}

[cla-assistant]

All committers have signed the CLA.

[sagikazarmark]

I wonder if the "old" config should be deprecated. Also, it might make sense to check if the client has any existing timeout and use that instead if so.

[liveFreeOrCode]

I think this makes sense, but I'm hesitant to do it as part of this PR. That is more of a design change rather than a feature to enable us the ability to start using our own HTTP client. Right now the schemaregistry.Config struct is mostly an opinionated abstraction over how an http.Client is configured, and I think I want to retain that intent through this work.

If later, it's desired to allow users to more explicitly set an http.Client and all it's settings, then that work should encompass everything from the TLS, timeout and any other settings currently exposed through schemaregistry.Config.

[sagikazarmark]

This is a requirement for proper observability, but context propagation would also be necessary (ie. add a context.Context first parameter to each method or introduce new SomethingContext variants to keep backwards compatibility)

[liveFreeOrCode]

This is a requirement for proper observability, but context propagation would also be necessary (ie. add a context.Context first parameter to each method or introduce new SomethingContext variants to keep backwards compatibility)

Good point. I'm leaning towards a version bump and just introducing ctx rather than FooContext for backwards compatibility, but bumping to v3 just for context propagation also seems like overkill. 🤔

[rayokota]

Subsumed by #1099