Add support for JWT Auth Method in Golang SDK

[vmaffet]

Problem

Currently, supported auth methods in the Go SDK are limited to:

• approle
• aws
• azure
• gcp
• kubernetes
• ldap
• userpass

The JWT method is missing from this list.

Prefered Solution

Add support for JWT under github.com/hashicorp/vault/api/auth so that it can be consumed easily.

i.e.

package main

import (
	"context"
	"fmt"

	vault "github.com/hashicorp/vault/api"
	auth "github.com/hashicorp/vault/api/auth/jwt"
)

func example(jwtToken string) (string, error) {
	client, _ := vault.NewClient(nil)

	jwtAuth, err := auth.NewJWTAuth(
		jwtToken,       // JWT token
		"dev-role-jwt", // Vault role
	)
	if err != nil {
		return "", fmt.Errorf("unable to initialize JWT auth method: %w", err)
	}

	authInfo, err := client.Auth().Login(context.Background(), jwtAuth)
	[...]
}

Alternatives

1. Use https://github.com/hashicorp/vault-client-go which supports it (Beta library - NOT for production)
2. Code the auth method yourself in a private package.

Additional context

This would make login through GitHub Actions OIDC more straightforward.