This commit introduces `CodecCustomizer`, a new callback-based interface
for customizing the codecs configuration for WebFlux server and client.

Instances of those customizers are applied to the `WebClient.Builder`
and to the `WebFluxAutoConfiguration` (which deals with both WebFlux and
WebFlux.fn).

For now, only Jackson codecs are auto-configured, by getting the
`ObjectMapper` instance created by Spring Boot. Other codecs can be
configured as soon as WebFlux supports those.

Closes gh-9166

This commit adds a new customizer interface for applying
configuration changes to `WebClient.Builder` beans:
`WebClientCustomizer`.

The new WebClient auto-configuration will make available, as a
prototype scoped bean, `WebClient.Builder` instances.
Once injected, developers can use those to create `WebClient`
instances to be used in their application.

`WebClientCustomizer` beans are sorted according to their
`Order` and then applied to the builder instances.

Closes gh-9522

Closes gh-9665

Closes gh-9664

Closes gh-9663

Closes gh-9662

Closes gh-9661

Closes gh-9660

Closes gh-9659

Closes gh-9658

Closes gh-9657

Closes gh-9656

Closes gh-9646

Closes gh-9545

Closes gh-9589

Closes gh-8026

Closes gh-9290

Upgrade to Bismuth snapshots

Closes gh-9580

Closes gh-9270

This commit adds ContextLoader, a test helper that configures an
ApplicationContext that is meant to simulate a particular
auto-configuration scenario.

The auto-configuration, user configuration and environment can be
customized. The loader invokes a ContextConsumer to assert the context
and automatically close the context once it is done.

Concretely, tests can create a shared field instance of that helper with
the shared configuration to increase its visibility and tune the context
further in each test.

If the context is expected to fail, `loadAndFail` allows to optionally
assert the root exception and consume it for further assertions.

This commit also migrates some tests to illustrate the practical use of
the helper

Closes gh-9634

Closes gh-9632

* pr/9633:
  Add GitHub link for Azure Spring Boot Starters

Closes gh-9633

Closes gh-9621

* pr/9628:
  Polish "Narrow the event types ConfigFileApplicationListener supports"
  Narrow the event types ConfigFileApplicationListener supports

Closes gh-9628