Declarative spring_starter.debug never enables span logging

Describe the bug

In declarative configuration mode, instrumentation/development/java/spring_starter/debug never
takes effect, so span logging cannot be enabled at all on that path.

Two things combine:

1. DeclarativeConfigLoggingExporterAutoConfiguration is missing from both auto-configuration
   registration files (AutoConfiguration.imports and spring.factories), so Spring Boot never
   discovers it and its DeclarativeConfigurationCustomizerProvider bean is never created. This is
   the same omission as #19725, which fixes the properties-mode counterpart.

2. Registering it is not sufficient. SpanLoggingCustomizerProvider.isEnabled reads the flag via
   the upstream SdkConfigProvider:

   return SdkConfigProvider.create(DeclarativeConfiguration.toConfigProperties(model))
       .getInstrumentationConfig("spring_starter")
       .getBoolean("debug", false);
   

   In the Spring starter, EmbeddedConfigFile builds the declarative model from
   Map<String, String>, so every scalar arrives as a String. getBoolean("debug", false)
   does not coerce, and returns false for a configured debug: true.

   That is exactly why this module already carries SpringConfigProvider and
   SpringDeclarativeConfigProperties, whose Javadoc says it "tries to coerce types, because
   spring doesn't tell what the original type was" — but isEnabled does not go through them.

Observed while working on #19725: with the class registered, the provider bean is created and
consumed, but the exporter is still not installed. Reading the same key directly shows
getString("debug") → "true" while getBoolean("debug", false) → false.

Steps to reproduce

Use the Spring Boot starter with declarative configuration and set
instrumentation/development/java/spring_starter/debug: true. No LoggingSpanExporter is
installed and no spans are printed.

Expected behavior

Enabling the declarative spring_starter.debug flag installs the logging span exporter, matching
the behaviour of otel.spring-starter.debug in properties mode.

Actual behavior

The flag is ignored. Even with the auto-configuration registered, isEnabled evaluates to false
because the configured value is a String and getBoolean does not coerce it.

Javaagent or library instrumentation version

main (verified at 4225fbe18e)

Environment

JDK 21, Spring Boot 3.2.4 / 2.6.15

Additional context

There seem to be at least three ways to fix this, and I did not want to pick one without
maintainer input, which is why #19725 deliberately leaves the declarative class out:

1. Have isEnabled read through the Spring-aware coercing provider
   (SpringConfigProvider / SpringDeclarativeConfigProperties) instead of SdkConfigProvider.
2. Preserve scalar types in EmbeddedConfigFile instead of flattening everything to String.
3. Make the upstream getBoolean coerce string values — SpringDeclarativeConfigProperties has a
   TODO pointing at open-telemetry/opentelemetry-java#8101, which looks related.

Whichever direction is preferred, the registration entries for
DeclarativeConfigLoggingExporterAutoConfiguration need to be added as part of it. Happy to
send a PR once the direction is settled.

Start with DeclarativeConfigLoggingExporterAutoConfiguration and compare its AutoConfiguration.imports and spring.factories registration with the properties-mode counterpart in #19725. Trace SpanLoggingCustomizerProvider.isEnabled through SdkConfigProvider, EmbeddedConfigFile, SpringConfigProvider, and SpringDeclarativeConfigProperties to confirm how the declarative debug value is read. Done means declarative spring_starter.debug: true installs LoggingSpanExporter and prints spans, with coverage for the supported configuration path.