Fixes gh-9693

Fixes gh-9351

This commit applies what's been done in gh-9166 for WebFlux client and
server, but for the `WebTestClient` auto-configuration.

`WebTestClient` can be configured for mock or integration tests and this
change applies `CodecCustomizer` beans to the client being built.

Closes gh-9577

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