Frontend: Backend connection for sign up page

[xltvy]

PR Description:

Our sign up page had appropriate form fields and validation check. However, the connection between the frontend of sign up page and our REST API has not been established yet.

This PR provides a trustfully functioning REST API connection between the sign up page and the appropriate endpoint.

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Notes:

• The endpoint used in the sign up form is auth/signup endpoint.
• Upon an unsuccessful response, the string in the response body's resultMessage field is displayed as an error message below the sign up button, with coherence to the validation error messages in the form fields.

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Routine Check

• I have fetched and pulled the latest version of the parent branch.
• This PR does not have any conflicts with the parent branch.

[gokayyildiz]

We can put this URL into a .env file and use it from that instead of using it statically on the code since we will need to use this value several times on the front-end project. Also, the backend will deploy the project several times and probably change the URL.

[gokayyildiz]

probably we will use it like;
await fetch("{process.env.BACKEND_URL}/auth/signup"

[Koraytkn]

We have checked for the above review and realized that "adding custom environment variables in react" could be another task instead of a fix for this issue, we will be adding the corresponding issue and initializing a config file in the future. Other than that @xltvy s implementation seems accurate, I am merging this branch to the frontend/dev and closing this pull request.