refactor(config): use envier for handling configuration

[P403n1x87]

Description

This is a first iteration on adopting envier to make configuration more declarative and 12-factor app-compliant.

Checklist

• [x] Title must conform to conventional commit.
• [x] Add additional sections for feat and fix pull requests.
• [x] Ensure tests are passing for affected code.
• [x] Library documentation and/or Datadog's documentation site is updated. Link to doc PR in description.

Reviewer Checklist

• [ ] Title is accurate.
• [ ] Description motivates each change.
• [ ] No unnecessary changes were introduced in this PR.
• [ ] PR cannot be broken up into smaller PRs.
• [ ] Avoid breaking API changes unless absolutely necessary.
• [ ] Tests provided or description of manual testing performed is included in the code or PR.
• [x] Release note has been added for fixes and features, or else changelog/no-changelog label added.
• [ ] All relevant GitHub issues are correctly linked.
• [ ] Backports are identified and tagged with Mergifyio.
• [ ] Add to milestone.

[mergify[bot]]

@P403n1x87 this pull request is now in conflict 😩

[codecov-commenter]

Codecov Report

Merging #3861 (fedff60) into 1.x (766702e) will increase coverage by 0.06%. The diff coverage is 93.85%.

@@            Coverage Diff             @@
##              1.x    #3861      +/-   ##
==========================================
+ Coverage   79.05%   79.12%   +0.06%     
==========================================
  Files         698      703       +5     
  Lines       54006    54206     +200     
==========================================
+ Hits        42697    42892     +195     
- Misses      11309    11314       +5     

Impacted Files	Coverage Δ
ddtrace/debugging/_snapshot/collector.py	83.78% <83.78%> (ø)
ddtrace/debugging/_probe/model.py	95.23% <94.73%> (-0.51%)	:arrow_down:
ddtrace/settings/config.py	88.73% <96.36%> (-3.34%)	:arrow_down:
ddtrace/bootstrap/sitecustomize.py	67.41% <100.00%> (+1.12%)	:arrow_up:
ddtrace/settings/agent.py	100.00% <100.00%> (ø)
ddtrace/settings/appsec.py	100.00% <100.00%> (ø)
ddtrace/tracer.py	89.77% <100.00%> (ø)
tests/debugging/probe/test_model.py	100.00% <100.00%> (ø)
tests/debugging/snapshot/test_collector.py	100.00% <100.00%> (ø)
... and 4 more

:mega: Codecov can now indicate which changes are the most critical in Pull Requests. Learn more

[mergify[bot]]

@P403n1x87 this pull request is now in conflict 😩

[mergify[bot]]

@P403n1x87 this pull request is now in conflict 😩

[mergify[bot]]

@P403n1x87 this pull request is now in conflict 😩

[P403n1x87]

Probably best to start from scratch with this at this point