GuzzleHttp library in Azure CDN provider will conflict with newer guzzlehttp/guzzle composer dependencies in other plugins

[figureone]

Aloha, W3 Total Cache includes an older GuzzleHttp library (v6.x, I believe) as part of the Microsoft Azure Storage CDN provider:
https://github.com/W3EDGE/w3-total-cache/tree/master/lib/Azure/GuzzleHttp

If W3 Total Cache is configured to use the Azure CDN provider, and if other installed plugins include the guzzlehttp/guzzle composer library v7.x or newer, a fatal error will be triggered when W3 Total Cache tries to load the older GuzzleHttp library. For example:
PHP Fatal error: Cannot redeclare GuzzleHttp\describe_type() (previously declared in /var/www/html/wp-content/plugins/authorizer/vendor/guzzlehttp/guzzle/src/functions.php:16) in /var/www/html/wp-content/plugins/w3-total-cache/lib/Azure/GuzzleHttp/functions.php on line 41
https://github.com/guzzle/guzzle/blob/master/src/functions.php#L5-L19

The root cause is the conditional check for the GuzzleHttp\uri_template function in functions_include.php:
https://github.com/W3EDGE/w3-total-cache/blob/master/lib/Azure/GuzzleHttp/functions_include.php#L4
Guzzle 7.x removed this function, so the conditional passes and tries to load the library, failing once it tries to redeclare the next function, GuzzleHttp\describe_type.

This should be a quick fix in W3 Total Cache (unless you want to move the entire dependency into composer, but I assume that could have side effects). Just check function_exists('GuzzleHttp\describe_type') instead of function_exists('GuzzleHttp\uri_template').
https://github.com/W3EDGE/w3-total-cache/blob/master/lib/Azure/GuzzleHttp/functions_include.php#L4

For reference, the guzzle folks rolled out a similar fix in 7.0.1:
https://github.com/guzzle/guzzle/pull/2699/files

Thanks!