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 89.89%. Comparing base (31869a3) to head (d7bcc24).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #7033   +/-   ##
=========================================
  Coverage     89.88%   89.89%           
  Complexity     6655     6655           
=========================================
  Files           748      748           
  Lines         20077    20081    +4     
  Branches       1969     1970    +1     
=========================================
+ Hits          18047    18052    +5     
  Misses         1435     1435           
+ Partials        595      594    -1     

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