4.x: Header.asString() and similar method lookup Mappers in service registry

[tomas-langer]

This is a performance issue.

Header.get() is OK, as it directly returns a string, all other methods cause a lookup in registry, which imposes a 20% performance degradation (i.e. Header.getString() is 20% slower than Header.get() )

This was identified when benchmarking declarative, though it is valid even for imperative code, as it is quite natural to call the typed method getString() instead of the generic one get()

[tomas-langer]

#10618 is a temporary fix that resolves a regression. This issue must fix the underlying problem of getting Mappers or MapperManager every single time parameters are created, or headers methods with mapping are called