Support decoding structs into Go maps

[alexejk]

As has been mentioned in #77, in some cases an XML-RPC struct member names are not known at compile time. This makes definitions of response datatypes much more complicated or not always possible.

To workaround this, StrDecoder can now accept map[string]any values alongside struct values.
In order to not over-complicate things a limitation on the map's key type to be a string has been added.

This new functionality should be backwards compatible as existing codebases could never use map datatypes for decoding, thus introducing additional flexibility to struct decoding won't cause any problems.

Fixes #77

[sonarcloud]

Quality Gate passed

Issues
0 New issues

Measures
0 Security Hotspots
93.8% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (2830010) 75.38% compared to head (abbf585) 75.60%.

Files	Patch %	Lines
decode.go	93.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #79      +/-   ##
==========================================
+ Coverage   75.38%   75.60%   +0.21%     
==========================================
  Files           7        7              
  Lines         524      541      +17     
==========================================
+ Hits          395      409      +14     
- Misses         99      101       +2     
- Partials       30       31       +1     

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