[Validator] Add the Yaml constraint

[javiereguiluz]

Fixes #19963.

Ping @xabbuh for help 🙏 I don't know how to express this --> YamlParser::PARSE_CONSTANT | YamlParser::PARSE_CUSTOM_TAGS | Yaml::PARSE_DATETIME in YAML and XML config formats. Can you help me? Thanks!

[alexandre-daubois]

I don't know how to express this --> YamlParser::PARSE_CONSTANT | YamlParser::PARSE_CUSTOM_TAGS | Yaml::PARSE_DATETIME in YAML and XML config formats

If I'm right, you cannot. You have to deal with the integer result of the bit operation. It is indeed not ideal.

Speaking of that, what would you think of a custom tag in the Yaml parser, like !flags YamlParser::PARSE_CONSTANT | YamlParser::PARSE_CUSTOM_TAGS? Would it make sense to you? I think it could be a nice addition to deal with flags in Yaml which seems still widely used in configs.

Edit: worked on something to achieve this: symfony/symfony#57522

[javiereguiluz]

Alex, thanks for reviewing this. For now, I removed the advanced flags example for YAML and XML. We can always re-add that later if your proposal is accepted. Thanks!