Magento – How to use token base Rest API

Anyway I have solved it myself.The captcha is not rendering due to I haven't override captcha layout xml so overriding it to your theme will solved issue.

I couldn't reproduce your error, but I got an other one.

Then I did some minor adjustments to your code and it worked.
First drop the InstallSchema.php file. It does nothing so it is useless.

Then make sure that your plugin file is in Amaj/CorePlugin/Model/Currency/Plugin.php and it looks like this:

<?php

namespace Amaj\CorePlugin\Model\Currency;

class Plugin
{
    protected $logger;

    public function __construct(\Psr\Log\LoggerInterface $logger) {
        $this->logger = $logger;
    }

    /**
     * @param $subject
     * @param \Closure $proceed
     * @param $price
     * @param array $options
     * @return string
     */
    public function aroundFormatTxt(
        $subject,
        \Closure $proceed,
        $price,
        $options = []
    )
    {
        $this->logger->info('Amaj aroundFormatTxt');
        return '2';
    }

}

you don't need parent::__construct(); in the constructor because there is no parent class.
And I replaced $logger->info with $this->logger->info because there is no $logger variable.