Drop Jetty from flume-ng-core (#446)

* Move HTTPSource to its own flume-http-source module

Keeping HTTPSource in flume-ng-core forced the heavy Jetty/Gson HTTP
stack onto every core consumer. Extract it into a dedicated optional
source module under flume-ng-sources, like taildir, so it ships only
when needed.

The package (org.apache.flume.source.http) is unchanged, so the
SourceType.HTTP reflective mapping and existing "http" configs keep
working. Core retains Jetty/Gson for its metrics server and
HTTPServerConstraintUtil.

Assisted-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Make flume-http-source self-contained for HTTP constraints

HTTPSource was reaching into flume-ng-core for HTTPServerConstraintUtil.
Move that helper into the module next to its only caller and make it
package-private, so the HTTP source no longer depends on a core internal
for its Jetty constraint handling. Declare jetty-security directly here
(managed in flume-parent) since the helper uses it.

Assisted-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Move monitoring services to flume-ng-instrumentation modules

Split the Ganglia, HTTP and Prometheus MonitorService implementations
out of flume-ng-core into three modules under a new flume-ng-instrumentation
parent, each in its own package so there are no split packages.

Replace the hardcoded MonitoringType enum with ServiceLoader discovery:
MonitorService gains a getType() default method, each provider declares
itself via META-INF/services, and the node selects one by matching
flume.monitoring.type against getType() (FQCN fallback preserved). The
modules are wired through the BOM and bundled by flume-ng-dist only.

Core keeps the MonitorService interface and JMXPollUtil, and drops the now
unused gson and prometheus dependencies.

Assisted-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Declare gson dependency in flume-taildir-source

The taildir source uses gson for its position file but was getting it
transitively from flume-ng-core, which no longer provides it after the
monitoring split. Without an explicit dependency the position file
handling fails at runtime with NoClassDefFoundError.

Assisted-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Drop Jetty from flume-ng-core

With the HTTP source and the HTTP/Prometheus monitors moved to their own
modules, nothing in flume-ng-core's main code uses Jetty anymore. Remove
the four jetty dependencies from the module.

flume-ng-node's TestHttpConfigurationSource was relying on Jetty arriving
transitively through flume-ng-core, so declare jetty-server and
jetty-servlet as test dependencies there.

Assisted-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>