Add base framework for functional configuration of Client. (#5)

## What changes are proposed in this pull request?

This PR adds the base framework for functional configuration of modular
API Clients (either REST or gRPC).

The principle behind that framework is to build a strong abstraction
between the configuration surface (i.e. the functional options) and the
actual config (which is internal). This allows to easily evolve and
extend the configuration surface without introducing breaking changes.

**Why functional options and not method on a struct with private
members?** It is more idiomatic and modular while ensuring that each
clients has its own immutable config.

## How is this tested?

No test yet, this PR is purely meant as a "demo" to be extended in an
actual implementation (e.g. by implementing a specific roundtripper for
the HTTP client).

Author: renaudhartert-db

databricks/internal/internal.go

@@ -0,0 +1,17 @@
+package internal
+
+import (
+	"net/http"
+	"time"
+
+	"github.com/databricks/sdk-go/auth"
+	"github.com/databricks/sdk-go/core/log"
+)
+
+type ClientOptions struct {
+	HTTPClient  *http.Client
+	Credentials auth.Credentials
+	Timeout     time.Duration
+	Logger      log.Logger
+	Debug       bool
+}

databricks/options/options.go

@@ -0,0 +1,48 @@
+package options
+
+import (
+	"net/http"
+	"time"
+
+	"github.com/databricks/sdk-go/auth"
+	"github.com/databricks/sdk-go/core/log"
+	"github.com/databricks/sdk-go/databricks/internal"
+)
+
+// ClientOption is an option to configure Databricks API clients.
+type ClientOption func(*internal.ClientOptions) error
+
+// WithHTTPClient returns a ClientOption to use a specific HTTP Client when
+// making HTTP requests.
+func WithHTTPClient(c *http.Client) ClientOption {
+	return func(o *internal.ClientOptions) error {
+		o.HTTPClient = c
+		return nil
+	}
+}
+
+// WithCredentials returns a ClientOption to set a specific credentials.
+func WithCredentials(c auth.Credentials) ClientOption {
+	return func(o *internal.ClientOptions) error {
+		o.Credentials = c
+		return nil
+	}
+}
+
+// WithTimeout returns a ClientOption to set the overall API call timeout to
+// the given duration by default.
+func WithTimeout(d time.Duration) ClientOption {
+	return func(o *internal.ClientOptions) error {
+		o.Timeout = d
+		return nil
+	}
+}
+
+// WithLogger returns a ClientOption to use the provided logger instead of
+// the default logger.
+func WithLogger(l log.Logger) ClientOption {
+	return func(o *internal.ClientOptions) error {
+		o.Logger = l
+		return nil
+	}
+}

databricks/transport/http.go

@@ -0,0 +1,26 @@
+package transport
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/databricks/sdk-go/databricks/internal"
+	"github.com/databricks/sdk-go/databricks/options"
+)
+
+func NewHTTPClient(ctx context.Context, opts ...options.ClientOption) (*http.Client, error) {
+	copts := &internal.ClientOptions{}
+	for _, opt := range opts {
+		if err := opt(copts); err != nil {
+			return nil, err
+		}
+	}
+
+	if copts.HTTPClient == nil {
+		copts.HTTPClient = http.DefaultClient
+	}
+
+	// TODO: Actually use the options...
+
+	return copts.HTTPClient, nil
+}