Introduce "dummy" Environment implementation

For testing purposes in which an Environment implementation is required
but a ConfigurableEnvironment is not desirable.

All methods are no-ops and return null, therefore NPEs are likely.
Relax ConfigurableWebEnvironment signatures

ConfigurableWebEnvironment was introduced in 3.2.0.M1 with SPR-9439 in
order to break a cyclic dependency. At the same time, certain signatures
such as AbstractRefreshableWebApplicationContext#getEnviroment and
GenericWebApplicationContext#getEnvironment were updated to take
advantage of covariant return types and return this newer, more narrow
type and providing cast-free calls to ConfigurableWebEnvironment methods
where necessary. Similar changes were made to HttpServletBean in
3.2.0.M2 with SPR-9763.

Narrowing #getEnvironment signatures in this fashion required enforcing
at the #setEnvironment level that any Environment instance provided
(explicitly or via the EnvironmentAware callback) must be an instance of
ConfigurableWebEnvironment. This is a reasonable assertion in typical
web application scenarios, but as SPR-10138 demonstrates, there are
valid use cases in which one may want or need to inject a non-web
ConfigurableEnvironment variant, e.g. during automated unit/integration
testing.

On review, it was never strictly necessary to narrow #getEnvironment
signatures, although doing so did provided convenience and type safety.
In order to maintain as flexible and backward-compatible an arrangement
as possible, this commit relaxes these #getEnvironment signatures back
to their original, pre-3.2 state. Namely, they now return
ConfigurableEnvironment as opposed to ConfigurableWebEnvironment, and in
accordance, all instanceof assertions have been removed or relaxed to
ensure that injected Environment instances are of type
ConfigurableEnvironment.

These changes have been verified against David Winterfeldt's Spring by
Example spring-rest-services project, as described at SPR-10138.

Issue: SPR-10138, SPR-9763, SPR-9439