Enable runtime substitutions using `class` level `validation` blocks

This fixes mapserver/mapserver/issues/4596.  As well as now honouring
class level validation blocks a validation hierarchy is implemented
across `web`, `layer` and `class` level validation blocks.  This
hierarchy *only* takes effect when identical validation keys appear in
`web`, `layer` or `class` such that keys in more specialised blocks
override those in more generalised blocks. i.e. `class` overrides
`layer` which overrides `web`.

In conjunction with the above, default class level substitutions are
also enabled and also implement the precedence rules above. i.e. a
default substitution in `class` overrides `layer` which overrides
`web`.