Add Cache File mode

[MGatner]

Description

• Adds support for configurable file modes when using the FileHandler for caching.
• Deprecates Config\Cache::$storePath in favor of a new driver-specific array of settings (similar to other drivers)
• Fixes a bug where '0' was not allowed as a cache prefix

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[MGatner]

Failure is unrelated.

[MGatner]

While moving things around I noticed that we use $this->prefix = $config->prefix ?: '' in all the handlers. Since Config\Cache::$prefix is already defined as a string all this essentially does is disables using falsey '0' as a prefix, so I've changed it out (still cast as a string in case someone has it incorrectly typed).