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 (31869a3) to head (9e09cd9).
Report is 19 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #7033      +/-   ##
============================================
+ Coverage     89.88%   89.96%   +0.07%     
- Complexity     6655     6668      +13     
============================================
  Files           748      748              
  Lines         20077    20121      +44     
  Branches       1969     1975       +6     
============================================
+ Hits          18047    18101      +54     
+ Misses         1435     1427       -8     
+ Partials        595      593       -2     

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[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.)