Add trace backends API

[florimondmanca]

Fixes #32, fixes #28.

Still TODO:

• Update docs.
• Validate on my personal website setup - see Refactor monitoring www#40 (doesn't actually use the backend API though).

[codecov-io]

Codecov Report

Merging #35 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #35   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          16     18    +2     
  Lines         399    460   +61     
=====================================
+ Hits          399    460   +61

Impacted Files	Coverage Δ
tests/test_backends.py	100% <100%> (ø)
src/ddtrace_asgi/_middleware.py	100% <100%> (ø)	⬆️
src/ddtrace_asgi/_backends.py	100% <100%> (ø)
src/ddtrace_asgi/__init__.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fde3c2e...78a186a. Read the comment docs.

[codecov-commenter]

Codecov Report

Merging #35 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #35   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           16        18    +2     
  Lines          399       460   +61     
=========================================
+ Hits           399       460   +61     

Impacted Files	Coverage Δ
src/ddtrace_asgi/__init__.py	100.00% <100.00%> (ø)
src/ddtrace_asgi/_backends.py	100.00% <100.00%> (ø)
src/ddtrace_asgi/_middleware.py	100.00% <100.00%> (ø)
tests/test_backends.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 78936f2...96a615b. Read the comment docs.