Add extension to allow AWS AppConfig to be used as a ConfigSource/ConfigSourceFactory

[brucej72gg]

There have been discussions in the past about providing a ConfigSource over AWS Secrets Manager & AppConfig, and Amazon have created a demo project here:

https://aws.amazon.com/blogs/mt/implementing-a-custom-configsource-in-quarkus-using-aws-appconfig-2/

However, this is very specific to simple configuration of a single project and not something that could be used as a general purpose extension. This extension aims to fix this limitation.

It is acknowledged that AppConfig may be problematic for CDI injected configuration in 'long running' applications, since Quarkus does not allow for dynamic configuration in this way (per https://quarkusio.zulipchat.com/#narrow/channel/187030-users/topic/ConfigSource.20for.20AWS.20AppConfig.20in.20Reactive.20approach)

However, in many cases (e.g. AWS Lambda) this would not be a problem, and there is a programmatic workaround. This clearly needs to be called out in the documentation for the extension.

[scrocquesel]

More people are wanting a config source extension fed by different aws services. Maybe we could come up with a general config source pattern. #1889

[melloware]

we should either add it as an extension here or add it to this existing one where HashiCorp Vault is: https://github.com/quarkiverse/quarkus-config-extensions