[#87] Add composer.json and composer.lock files for snyk test

Request: https://github.com/snowflakedb/snowflake-sdks-drivers-issues-teamwork/issues/87 Snyk requirements for PHP: https://docs.snyk.io/scan-application-code/snyk-open-source/language-and-package-manager-support/snyk-for-php

Files changed:

composer.json: This file describes the dependencies of the project and other metadata.
composer.lock: This file is generated from composer.json and locks the project to specific versions of all packages.
.gitignore: added the vendor folder as we don't want to add all of the third-party code to the repo repository. This is recommended by the composer docs. Composer docs: https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies

Snyk only supports Composer for PHP which requires the composer.json and composer.lock files to be present in the repository. However, the PHP driver is written in C and it only needs dependencies from libsnowflakeclient. Snyk test result will show 1 dependency only which is the minimum PHP version.

"require": {
    "php": ">=7.4"
}

Snyk test result:

C:\GitRepo\SNOW-665303-snyk-test\pdo_snowflake>snyk test

Testing C:\GitRepo\SNOW-665303-snyk-test\pdo_snowflake...

Organization:      sfc-gh-ext-simba-jl
Package manager:   composer
Target file:       composer.lock
Project name:      snowflakedb/pdo_snowflake
Open source:       no
Project path:      C:\GitRepo\SNOW-665303-snyk-test\pdo_snowflake
Licenses:          enabled

✔ Tested 1 dependencies for known issues, no vulnerable paths found.

snowflakedb / pdo_snowflake

[#87] Add composer.json and composer.lock files for snyk test #307