Closes gh-5075

Previously, Spring Boot mapped both `DataSourceProperties` and the actual
`DataSource` implementation to the same prefix. This results in a huge
amount of keys in the `spring.datasource` namespace  with no way to
identify those that are valid for the pooled data source in use.

This commit maps the four pooled data sources we support in four isolated
namespace, keeping `spring.datasource` only for the common settings.

These are `spring.datasource.tomcat`, `spring.datasource.hikari`,
`spring.datasource.dbcp` and `spring.datasource.dbcp2` for the Tomcat,
Hikari, Commons DBCP and Commons DBCP2 implementations respectively.

Closes gh-2183

Closes gh-5025

* pr/5060:
  Add ref to Google gRPC starter

Closes gh-5060

* pr/5067:
  Fix typos

Closes gh-5067

Spring Boot fires event very early in the application lifecycle and we
should make crystal clear that a regular `@Bean` registration cannot be
used to register a listener on them.

Closes gh-5061

Ensure that JarFile caches are cleared once the ApplicationContext has
loaded. Caches are cleared manually with the assumption that no
further class loading is likely.

Closes gh-4882

Refactor `spring-boot-loader` to reduce the amount of memory required
to load fat & exploded jars. Jar files now no longer store a full list
of entry data records, but instead use an array of entry name hashes.

Since ClassLoaders often ask each JAR if they contain a particular
entry (and mostly they do not), the hash array provides a quick way to
deal with misses. Only when a hash does exist is data actually loaded
from the underlying file.

In addition to the JarFile changes, the Archive abstraction has also
been updated to reduce memory consumption.

See gh-4882

Remove the use of JDK loggers in Launcher and PropertiesLauncher to
ensure allow the custom Log4j2 `LogManager` to be used with Spring Boot
applications.

Fixes gh-3815

Update SpringApplication to allow classes loaded from spring.factories
to be package private.

Fixes gh-5043

Add SharedMetadataReaderFactoryContextInitializer to ensure that a
shared caching MetadataReaderFactory is used between configuration
classes and auto-configure sorting.

Fixes gh-4993

* contributor_covenant:
  Add contributor covenant

Closes gh-4914

This commit enables the use of // @formatter:off and // @formatter:on
to surround lines of code that should not be formatted. It also adds
a code template that can be used to quickly wrap the selected lines
of text with the off and on comments.

Closes gh-5050

* pr/5026:
  Add cloudant-spring-boot-starter reference

Closes gh-5026

Closes gh-5041

jetty-jndi is no longer shipped by default as of 2b2e21db but one test
checking for this wasn't updated properly.

* pr/4411:
  Update copyright header
  Remove closure-execution from OptionHandler

It seems that the code for executing a groovy closure from the
OptionHandler is never executed and therefore not needed.

Removing the code gives the benefit that the Groovy-classes are not
needed if someone else wants to use the spring-boot-cli infrastructure to
run his own cli interface.

Closes gh-4411

Closes gh-4479

Closes gh-4482

Closes gh-4490

Our Investigations show that JNDI lookup is quite costly on Jetty and it
was enabled by default while embedded tomcat does not. This commit
excludes `jetty-jndi` to align the behavior with Tomcat.

If you are relying on JNDI, the `jetty-jndi` should simply be added in
your project.

Closes gh-4710

The `DataSource` auto-configuration in Spring Boot supports two modes:
regular pooled DataSource and embedded database (via the
`EmbeddedDatabase` infrastructure provided by `spring-jdbc`. These were
previously named `NonEmbedded` and `Embedded` respectively.

This commit clarifies those mode and in particular that a pooled
`DataSource` can also handle an embedded database.

Closes gh-4634

* pr/5028:
  Added dependency for jackson-datatype-json-org

Closes gh-5028

Introduce dedicate method to determine the actual value to use based on
the configuration and the environment.

Closes gh-1436

* pr/4574:
  Polish contribution
  Support for SendGrid ApiKey