Add support for CETL VariableLengthArray and polymorphic_allocator

[skeetsaz]

This change modifies the C++ code-gen to create message classes that can be used with allocators.

• The message class becomes an "_Impl" template that takes an allocator, which gets passed down to any variable length arrays in the messages (or in contained messages.) The normal message name is then aliased to the "_Impl" template using the allocator specified in the yaml config. In the end the message is a regular struct as it was before.
• The old variable_length_array.hpp is removed from the support library. CETL is added as a submodule and the new variable_length_array.hpp from there is used.
• Added a verification test to exercise a message using CETL's polymorphic_allocator

[thirtytwobits]

You need to run Black to get the CI past the formatting errors.

[skeetsaz]

Updated my pull-request per the feedback. The ctor_convention approach and -std=cetl++ -std=c++17-pmr flags help simplify things and maintain better backward compatibility.

The verification tests I added will fail due to CETL VLA not implementing the trailing-allocator convention correctly (I filed OpenCyphal/CETL#57 for that.) But I want to get this out for more feedback.

[thirtytwobits]

Is there a way to specify the default memory resource that I'm missing?

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
1 Code Smell

86.4% Coverage
0.0% Duplication