Add experimental option to allow stack extension

[agoallikmaa]

Resolves open-telemetry/opentelemetry-php#1223

Add INI option opentelemetry.allow_stack_extension (default Off) that enables experimental stack extension.

When processing the return value of a pre hook, the number of slots that the stack can be extended by is calculated. If the INI option is Off, this will be 0. Otherwise it is the minimum of remaining slots in the stack page and a hardcoded limit of 16. The purpose of the limit is to allow collecting the extended values to a temporary array without explicit allocations, so that the changes to stack size (and moving around slots on the stack) could be done in one step at the end.