Add startup Actuator endpoint

This commit builds on top of gh-22603 and exposes data collected by the
`BufferingApplicationStartup` on a dedicated `"/startup"` Actuator
endpoint.

Closes gh-23213

spring-boot-project/spring-boot-docs/src/docs/asciidoc/production-ready-features.adoc

@@ -121,6 +121,10 @@ The following technology-agnostic endpoints are available:
 | Lets the application be gracefully shutdown.
   Disabled by default.
 
+| `startup`
+| Shows the startup steps data collected by the `ApplicationStartup`.
+  Requires the `SpringApplication` to be configured with a `BufferingApplicationStartup`.
+
 | `threaddump`
 | Performs a thread dump.
 |===
@@ -273,6 +277,10 @@ The following table shows the default exposure for the built-in endpoints:
 | Yes
 | No
 
+| `startup`
+| Yes
+| No
+
 | `threaddump`
 | Yes
 | No