Introduce a strategy to `WebApplicationType` to allow modules to
implement deduction logic.
Prior to this commit, modules played no part in deducing the
`WebApplicationType`. This meant that a user with `spring-webflux`
for client purposes would deduce `REACTIVE` despite no
`spring-boot-webflux` module being present.
The following deduction logic order is now implemented:
1) If the `spring-boot-webmvc` module is being used and Spring MVC
classes are found then `SERVLET` is used.
2) If the `spring-boot-webflux` module is being used and Spring WebFlux
classes are found then `REACTIVE` is used.
3) If `spring-web` is found and servlet classes are available then
`SERVLET` is used.
4) If none of the above are satisfied, `NONE` is used.
This commit also updates `SpringBootTestContextBootstrapper` to use
the same deduction logic.
Fixes gh-48517
Phillip Webb