* simplify placeholders
* update placeholders_test.py
* update structures to use placeholders correctly
* update use of placeholders elsewhere in codebase
* clean up
* move placeholder type checking logic to PipelineTask to prevent v1 load breaks
* clean up
* update docstrings
* make to_dict and to_string private methods
* revert unnecessary change to tests
* improve type safety of code
* prefix module-level constant with underscore
* remove dependency of deprecated on v2
* use new artifact identification logic
* add custom artifact type compiler test
* compile schema_version from non-kfp artifacts
* update tests
* add importer to compiler test
* update golden snapshots
* fix type checking code
* undo executor_test diff
* remove gcpc types compiler test
* add compilation test comment
* update google artifact handling
* update compiler test
* only permit system. and google. artifact types; add tests
* update compiler test
* simplify type checking logic
* use bundled artifact types in PipelineArtifactChannel
* raise error on unknown v1 input or output
* add type utils and move functions
* add type utils tests
* remove schema_version from inputspec and outputspec
* update artifact proto creation and tests
* propogate bundled artifact type string changes throughout sdk
* update golden snapshot
* remove unused v1 compat and tests
* update type checking and tests to validate schema_version using semantic versioning
* update importer sample
* update test data config
* update test_data config
* clean up sample
* clean up sample
* test if and concat placeholder and validate artifact channel usage
* fix nit in read write test config
* resolve comments
* resolve comments and move test cases
* resolve comments and add placeholder validation
* remove unused imports
* update release file
* move placeholders to placeholders module; implement if-present and concat placeholders
* update module name and method names throughout codebase
* rename placeholder abc
* add back compat support for CEL-style concat string
* add copyright
* add docstrings; clean up
* remove duplicate code