Update `composer.json` with `ext-gmp` dependency

[jerowork]

Description

In order to make use of the gmp_* functions, PHP needs to have GMP installed (not installed by default).

More information about GMP: https://www.php.net/manual/en/book.gmp.php

While installing the GMP library is not part of this package, it is a common use to add external dependencies to the composer.json as requirement.

Without the GMP extension, it cannot find the needed functions, resulting in fatal errors like:

Call to undefined function EllipticCurve\Utils\gmp_init()

(there is a opened issue at starkbak/sdk-php for it: https://github.com/starkbank/sdk-php/issues/66)

Changes

• [x] Added "ext-gmp": * as requirement to composer.json
• [x] Updated README installation details with external dependency requirements

[xavier-stark]

Hi @jerowork,

Thanks for submitting this pull request to help us fix the issue.

We have reviewed your changes and will be merging them into our main branch shortly.

Thanks again for your contribution! If you have any other ideas or feedback to improve our project, please feel free to share them with us.

Best regards, Xavier