Add a `ConfigurationPropertyCaching` utility interface that can be
used to control the property source caching.

Prior to this commit, a `ConfigurationPropertySource` that was backed
by a mutable `EnumerablePropertySource` would need to call the
`getPropertyNames()` method each time a property was accessed. Since
this this operation can be expensive, we now provide a way to cache
the results for a specific length of time.

This commit also improves the performance of immutable property sources
by limiting the number of candidates that need to be searched.
Previously, all mapped names would be enumerated. Now, mappings are
grouped by `ConfigurationPropertyName`. This is especially helpful when
the `ConfigurationPropertyName` isn't mapped at all since the hash based
map lookup will be very fast and the resulting mappings will be empty.

Closes gh-20625

Provide a hashcode implementation for `ConfigurationPropertyName` so
that instances can be stored in Map without them all ending up in the
same bucket.

See gh-20625

Add a test to ensure that a large number of property sources that each
contain many items can perform well.

See gh-20625

Remove the recently added slash restriction since Spring Cloud
Config Server needs to support names with slashes.

See gh-21217

Closes gh-21352

Closes gh-21351

Closes gh-21342

Closes gh-21349

Closes gh-21348

Closes gh-21346

Closes gh-21344

Closes gh-21213

Closes gh-21343

Prior to this commit, the property was a Boolean with a null default.
If it was explicitly set by the user, a context customizer would use
that value to set it on the context. However, if it was not set, the default
wouldn't be tomcat's default but `false` because it was explicitly set to
`false` in `TomcatServletWebServerFactory`. This commit defaults the property
itself to `false` so that the default is more obvious to the user.

Fixes gh-20796

Fix the `SystemEnvironmentPropertyMapper.isAncestorOf` implementation
to convert names based on their dashed form and to silently ignore
any invalid results.

Closes gh-14479

Closes gh-21341

Fixes gh-20759

The warnings will be addressed by gh-20759. CreateBootStartScripts
must be excluded from the classes that are validated by the
ValidatePlugins task. It is invalid, but only for Gradle 6.4. gh-20759
will cause it to only be used with Gradle 6.3 and earlier.

See gh-21329

Closes gh-21340

Closes gh-21339

Closes gh-21338

Closes gh-21337

Closes gh-21336

Closes gh-21335

Closes gh-21334

Closes gh-21333

Closes gh-21332

Closes gh-21331

Closes gh-21330

Closes gh-21329