…#8080)

This pull request makes `Tracer.enabled` configurable via remote.
Semantically this configuration is a one-way switch: it can be remotely
set to False, but once it's False it cannot be remotely set to True.
When False, it can be set to True via code or environment variable.

There isn't much risk in this change because it's a no-op until the UI
starts sending the relevant remote configuration flag.

The remote config backend change supporting this feature is dd-go PR
118200.
The system-tests change validating this new functionality is
DataDog/system-tests#2039.

## Checklist

- [x] Change(s) are motivated and described in the PR description.
- [x] Testing strategy is described if automated tests are not included
in the PR.
- [x] Risk is outlined (performance impact, potential for breakage,
maintainability, etc).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
are followed. If no release note is required, add label
`changelog/no-changelog`.
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/)).
- [x] Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))
- [x] Check against client capabilities documentation
- [x] system-tests added here
https://github.com/DataDog/system-tests/blob/37f0b61616884cbb8dd553415409c1ad0152ad98/tests/parametric/test_dynamic_configuration.py

## Reviewer Checklist

- [x] Title is accurate.
- [x] No unnecessary changes are introduced.
- [x] Description motivates each change.
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
- [x] Testing strategy adequately addresses listed risk(s).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] Release note makes sense to a user of the library.
- [x] Reviewer has explicitly acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment.
- [x] Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)
- [x] If this PR touches code that signs or publishes builds or
packages, or handles credentials of any kind, I've requested a review
from `@DataDog/security-design-and-guidance`.
- [x] This PR doesn't touch any of that.

APMON-578

---------

Co-authored-by: Tahir H. Butt <tahir.butt@datadoghq.com>