api: conditionally load entrypoints for OTEL_PYTHON_CONTEXT (#5144)

* api: conditionally load entrypoints for OTEL_PYTHON_CONTEXT

This prevents unnecessarily importing the entry points library, reducing
the memory footprint of loading the API by about a MB (27%):

|Scenario | Memory before | Memory after | delta |
| - | - | - | - |
|Import API only | 5.43 MB | 4.49 MB | -940 KB |
|API + get_tracer (no-op) | 5.05 MB | 4.10 MB | -950 KB |

Signed-off-by: Alex Boten <223565+codeboten@users.noreply.github.com>

* add ignore rule to import lint, update changelog

Signed-off-by: Alex Boten <223565+codeboten@users.noreply.github.com>

* fix precommit

Signed-off-by: Alex Boten <223565+codeboten@users.noreply.github.com>

* clean up changelog

Signed-off-by: Alex Boten <223565+codeboten@users.noreply.github.com>

* use changelog fragment

Signed-off-by: Alex Boten <223565+codeboten@users.noreply.github.com>

* apply the same pattern to propagators and providers

Signed-off-by: Alex Boten <223565+codeboten@users.noreply.github.com>

* precommit

Signed-off-by: Alex Boten <223565+codeboten@users.noreply.github.com>

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* put public symbols back

Signed-off-by: Alex Boten <223565+codeboten@users.noreply.github.com>

* rename vars

Signed-off-by: Alex Boten <223565+codeboten@users.noreply.github.com>

---------

Signed-off-by: Alex Boten <223565+codeboten@users.noreply.github.com>
Co-authored-by: Mike Goldsmith <goldsmith.mike@gmail.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>