… PHPs < 8.0 (#1211)

Most of references to force_current_segment or nr_txn_force_current
segment were placed in if/endif blocks with the OAPI effort. The change
was never explicitly mentioned and there's a chance it could be used in
a breaking manner. All remaining references in axiom specify explicitly
it is not to be used with oapi. Axiom doesn't understand ZEND_API
numbers so it can't be placed in if/endif blocks.

1) Updates `nr_txn_get_current_segment` usages to also incorporate new
get_current_segment calls.
2) New functions added are:
 `nr_txn_get_current_segment_txn_context` (in axiom)
`nr_php_txn_get_current_segment_php_context` (in agent since axiom is
php agnostic)
3) Added additional integration tests

Analyze all segment starts to work with context information.
Will behave as usual for non-fibers.
Will use context information for starting fibers when in a fiber
environment.

1) Updated segment start logic across the agent to be context aware
2) Added fiber integration tests to exercise the code paths
3) Incorporated bug fixes that surfaced during increased test coverage
runs.


Of the tests:
* The _basic tests all follow the same pattern.
* For the pdo/*/base-class tests, once you've seen one of the variants,
the others are basically the same but for different databases.
* the span_events/fibers/*.php tests going through various scenarios
(starting/stopping/resume/noresume/errors/etc/nesting) etc.
* guzzle7 and curl_multi_exec are actually async so there's quite a few
more tests in that area.

## Summary

Each PHP Fiber now gets its own snapshot of the request-scoped globals
the
agent relies on (`ctx`), so instrumentation running inside
a fiber no longer trips over state owned by the main context (or another
fiber). The fiber observer hooks already in place from the parent branch
are wired up to populate, swap, and tear down these snapshots; the
`NRPRG_CTX` accessor transparently reads from the active
fiber's snapshot when one is set.

All new behavior is gated on PHP 8.1+. 

## What changed

**Per-fiber globals (`agent/php_fibers.{c,h}`, new)**
- New `fiber_globals_t` = `{ ctx_globals_t* }`.
- `nr_fiber_copy_ctx_globals` deep-copy the parts a fiber should own. 
- A per-request `fiber_globals_map` (keyed by `"%p"` of the
`zend_fiber_context*`) holds these snapshots; it has a destructor so
removed
entries are fully cleaned up. Init is lazy on first fiber; teardown
happens
in `nr_php_post_deactivate`. `php_rshutdown.c` clears
`NRPRG(fiber_globals)`
before normal global teardown so the existing free paths don't reach
into a
  snapshot.

**OAPI fiber observer wiring (`agent/php_observer.c`)**
- `nr_fiber_init_observe`: lazy-init `fiber_globals_map`, add a snapshot
for
  the new fiber under its `"%p"` key.
- `nr_fiber_destroy_observe`: remove the snapshot for that fiber.
- `nr_fiber_switch_observe`: at the end of the existing switch logic,
swap
  `NRPRG(fiber_globals)` to the destination context's snapshot via
  `nr_fiber_switch_global_context`.

**Globals accessors (`agent/php_newrelic.h`)**
- New `fiber_globals_t` struct; `NRPRG(fiber_globals_map)` and
  `NRPRG(fiber_globals)` added to module globals.
- On PHP 8.1+, `NRPRG_CTX(Y)` resolves through
`NRPRG(fiber_globals)` when it's non-NULL, otherwise read the
main-context
globals exactly as before. Callers don't have to know which context
they're
  in. PHP < 8.1 macros are unchanged.

---------

Co-authored-by: Amber Sistla <asistla@newrelic.com>