feat: add CreateAccount call

[chuhlomin]

This PR adds implementation for POST /accounts request that creates new account and assigns plan keys to it.

Example usage:

ctx := context.Background()
client := kickplan.NewClient()

// Create new account with "free" and "pro" plans
err := client.CreateAccount(ctx, "key", "Name", "free", "pro")
if err != nil {
	log.Fatalf("failed to create account: %v", err)
}