feat: add convert argv parser

[guilhhotina]

summary

• add a boost.spirit x3 parser for convert argumnts and the ast
• turn the parsed command into haio pipeline steps
• keep the cli close to the c version, but use the c++ crop/resize suport where it already exists
• cover the parser cases from the c branch and a few c++ extras

tests

• cmake --build build-cdn -j 16
• ctest --test-dir build-cdn --output-on-failure
• manual convert smoke tests for crop, resize, format prefixes, stdin/stdout, and unsupported generator/fx paths