[PHP] Optimise building cache

[itscaro]

Expected Behaviour

When dependencies/requirements are not changed, these build steps should be cached. Pros:

• build time is reduced
• reduce Github rate-limit issues (when not providing a Github token) Cons:
• composer.json and php-extensions.sh must exist for this template, the user cannot delete them when they are not used

Current Behaviour

Any modification in function directory uncaches the build step starting from copy of function source codes.

[itscaro]

Derek add label: proposal

[itscaro]

Derek add label: php