Mappings expand <namespace> parameter

[isc-pbarton]

Mapping configurations may now include the <namespace> token. The token will be expanded to the name of the current namespace. This is the first step towards supporting an instance-wide monorepository in which custom code for each namespace is mapped to a different subdirectory.
Resolves #710 and #756

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 41.33%. Comparing base (88fb29b) to head (6209c3a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #762      +/-   ##
==========================================
+ Coverage   41.30%   41.33%   +0.03%     
==========================================
  Files          24       24              
  Lines        3213     3215       +2     
==========================================
+ Hits         1327     1329       +2     
  Misses       1886     1886              

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