Add support for escaping declarative config env var substitution

[jack-berg]

Implementation of spec PR: open-telemetry/opentelemetry-specification#4375

# given STR=val
key: ${STR}                           # yields key: val
key: ${UNKNOWN:-val}        # yields key: val
key: $${STR}                         # yields key: ${STR}   <-- NEW
key: $${UNKNOWN:-val}      # yields key: ${UNKNOWN:-val} <-- NEW

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.96%. Comparing base (9cb6365) to head (a17bd3e).
Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #7033      +/-   ##
============================================
+ Coverage     89.92%   89.96%   +0.03%     
- Complexity     6721     6723       +2     
============================================
  Files           765      765              
  Lines         20277    20293      +16     
  Branches       1985     1990       +5     
============================================
+ Hits          18234    18256      +22     
+ Misses         1448     1441       -7     
- Partials        595      596       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[tsloughter]

Oh, now I see the collector uses $$. Then I suppose this is the way to go. May want to also mention additional escaping like allowing \$${DOLLARS} to be $10?

(Oops, had too many tabs open. This was supposed to be on the issue in the spec.)

[trask]

Thanks for reviewing @jaydeluca!