chore: [SVLS-5262] move private Span Pointers to a new file

[apiarian-datadog]

In preparation for future work, let's move the private span pointer code out to its own file.

Checklist

• PR author has checked that all the criteria below are met
• The PR description includes an overview of the change
• The PR description articulates the motivation for the change
• The change includes tests OR the PR description describes a testing strategy
• The PR description notes risks associated with the change, if any
• Newly-added code is easy to change
• The change follows the library release note guidelines
• The change includes or references documentation updates if necessary
• Backport labels are set (if applicable)

Reviewer Checklist

• Reviewer has checked that all the criteria below are met
• Title is accurate
• All changes are related to the pull request's stated goal
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Newly-added code is easy to change
• Release note makes sense to a user of the library
• If necessary, author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[github-actions]

CODEOWNERS have been resolved as:

ddtrace/_trace/_span_pointer.py                                         @DataDog/apm-sdk-api-python
ddtrace/_trace/_span_link.py                                            @DataDog/apm-sdk-api-python
ddtrace/_trace/span.py                                                  @DataDog/apm-sdk-api-python
tests/tracer/test_encoders.py                                           @DataDog/apm-sdk-api-python
tests/tracer/test_span.py                                               @DataDog/apm-sdk-api-python

[datadog-dd-trace-py-rkomorn]

Datadog Report

Branch report: aleksandr.pasechnik/svls-5262-1-extract-span-pointers
Commit report: 4534d8c
Test service: dd-trace-py

✅ 0 Failed, 1286 Passed, 0 Skipped, 37m 7.16s Total duration (1m 21.48s time saved)

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-09-19 13:54:30

Comparing candidate commit 4534d8c in PR branch aleksandr.pasechnik/svls-5262-1-extract-span-pointers with baseline commit aef5bc0 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 319 metrics, 47 unstable metrics.