Validate directive names in configuration

Problem/Motivation

The Merge Request for 📌 Add validation constraints to core.extension Needs review includes a SequenceKeys constraint that would allow validating the directive names in config

csp_policy:
  mapping:
    directives:
      constraints:
        SequenceKeys:
          constraints:
            Choice:
              callback: \Drupal\csp\Validation\DirectiveNames::values

Proposed resolution

Add a config validation constraint that checks directive sequence keys against Csp::DIRECTIVES

Remaining tasks

User interface changes

API changes

Data model changes