Schema: Support conditional subschemas

Description of feature

It is quite common to want to have conditional validation for pipeline parameters. Currently, this is not supported and must be done purely within the pipeline code. However, native JSON schema can support it with two properties:

• dependentSchemas
  • If params.foo is defined at all, then additional validation rules / required parameters for params.bar should apply.
  • Typical use case: if --do_this is enabled, then --this_option must be set
• If-Then-Else
  • If params.foo matches a specific set of rules, then additional validation rules / required parameters for params.bar should apply.
  • Typical use case: if --method is set to --foo, then --foo_method_option must be set

Start with the conditional-schema requirements in the issue and the linked JSON Schema references. Trace how nf-core/tools currently validates pipeline parameters; done means dependentSchemas and if/then/else rules work for the described cases and are covered by validation tests.