Provide extensions that remove the need for the log4j-spring-boot log4j2 module

[rgoers]

This change merges the contents of the log4j-spring-boot module into Spring Boot, effectively making Log4j 2 on par with Logback and eliminating the need for the log4j-spring-boot module.

I was able to verify this with a sample project I have.

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The following sub-tasks have been created based on this PR:

• Add support 'log4j.configurationFile' system property #32730
• Add Spring Environment to Log4j2 LoggerContext #32731
• Add Log4j2 StrLookup backed by the Spring Environment #32732
• Add Log4J2 PropertySource backed by the Spring Environment #32733
• Support profile specific Log4j2 configuration #32734
• Log4J2 configuration URLs should respect system settings #32735

[philwebb]

Thanks very much @rgoers, and thanks for your patience whilst we go to this. I've split the PR into a number of distinct issues so that our automated changelog is correct.

I made a few amendments to some of the functionality, namely:

• I removed the ability in StrLookup to resolve profiles.active and profiles.enabled. This simplifies quite a bit of code and I can't think of a good reason why users should need to resolve the profiles.
• A few of the extensions now fail hard if the Environment isn't available when they're used
• URL based lookups must resolve a config and if they don't the application will fail to start

I've also added a new section in the reference guide to explain the features.

Thanks again for the contribution and for all your hard work on Log4j2!