feat: add support for using $_ENV instead of getenv/putenv

[bshaffer]

getenv and putenv are considered not threadsafe. As a result,
We should expose a way (similar to symphony's usePutenv function) to use the threadsafe version ($_ENV) instead.

As @DracoBlue mentioned in #429:

The current version of google auth library php uses getenv/putenv to read and set environment variables in php. But the function putenv is not threadsafe, which made libraries (Dotenv https://github.com/symfony/dotenv/blob/6.2/Dotenv.php#L61 and phpdotenv https://github.com/vlucas/phpdotenv#putenv-and-getenv) to disable putenv support by default. Frameworks like symfony (deprecated in 4.3 symfony/symfony#31062 and removed in 5.0) and also in laravel there was a similiar solution to not use it anymore directly (vlucas/phpdotenv#76).

Thus here is a pull request to change all the ocurences of:

• getenv($variable) to array_key_exists($variable, $_ENV) ? $_ENV[$variable] : false
• putenv($variable) to unset($_ENV[$variable])
• putenv($variable . "=") to $_ENV[$variable]=''
• putenv($variable . "=" . $value) to $_ENV[$variable]=$value

Threads about this:

• https://bugs.php.net/bug.php?id=74143 (putenv does not work if fast cgi sets the variable!)
• [Dotenv] Deprecate useage of "putenv" symfony/symfony#31062
• Changing process environment unsafe on multithreaded servers vlucas/phpdotenv#76
• .env issue with PHP thread-safe laravel/framework#7354
• laravel/framework@866a8cf
• BUG Replace phpdotenv with thread-safe replacement silverstripe/silverstripe-framework#7484
• Laravel 5.8 update breaks third-party composer libraries laravel/framework#27949

[DracoBlue]

Thanks for creating the feature request!