Mappable values

[tomas-langer]

Resolves #1897 (mappable query params and path params) Related to #1789 (align mappers)

Added Value and ValueProvider to mapper module. It is now used in various HTTP classes (Headers, query parameters etc.) and in Config (ConfigValue).

Added support for a shared MapperManager that can be custom registered by user. Mapping to basic types is built-in (String -> a lot of types), including date types. Format can be overridden through context if desired (see MapperManager.FORMAT_CLASSIFIER).

The changes are fully backward compatible - we have to decide how to approach this (e.g. do we want to keep all the methods, or do we want to change signatures in the future?).

Created as a draft until we agree on approach.

[romain-grecourt]

It would be easier to have a markdown document going over the proposal. We should describe all the various use-cases with snippets:

• config
• dbclient
• media support
• webserver query params

Highlight the complex type usages with GenericType, document the special layering of media-support.

We should also have an example that demonstrates the value of this API when combining dbclient and webserver/webclient. E.g. Show how to support a custom entity all the way from a request payload to the database.

[tomas-langer]

I am adding a docs-internal/generic-mapping-2.md