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

Closes gh-23213

This commit adds a new tag to the `"spring.boot.application.starting"`
startup step.

Closes gh-23162

As of spring-projects/spring-framework#24878, Spring Framework provides
an `ApplicationStartup` infrastructure that applications can use to
collect and track events during the application startup phase.

This commit adds a new `BufferingApplicationStartup` implementation that
buffer `StartupStep`s and tracks their execution time. Once buffered,
these steps can be pushed to an external metrics system or drained
through a web endpoint, to a file...

Closes gh-22603

Closes gh-23212

Closes gh-23211

Closes gh-23208

Closes gh-23207

See gh-23188

See gh-23189

See gh-23197

See gh-23199

See gh-23187

See gh-23196

See gh-23192

See gh-23198

See gh-23186

See gh-23195

See gh-23191

See gh-23204

See gh-23203

See gh-23036

See gh-23185

See gh-23194

See gh-23190

See gh-23181

Closes gh-23205

Closes gh-23200

See gh-19471

Closes gh-23204

Closes gh-23203

Closes gh-23201

Closes gh-21122

Fixes gh-23036

See gh-23183

See gh-23182