Add parser config to load external dtd to resolve entities

[tefra]

📒 Description

Loading external dtd is supported by lxml we should expose this in the parser config.

Fixes #769

🔗 What I've Done

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💬 Comments

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🛫 Checklist

• Updated docs
• Added unit-tests
• Sample tests pass
• W3C tests pass

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (a768eef) 99.96% compared to head (d75972e) 99.96%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #797   +/-   ##
=======================================
  Coverage   99.96%   99.96%           
=======================================
  Files         104      104           
  Lines        9208     9209    +1     
  Branches     2060     2060           
=======================================
+ Hits         9205     9206    +1     
  Partials        3        3           

Impacted Files	Coverage Δ
xsdata/formats/dataclass/parsers/config.py	100.00% <100.00%> (ø)
xsdata/formats/dataclass/parsers/handlers/lxml.py	100.00% <100.00%> (ø)

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