[Config] Add Disable Send and Context Propagation Only

[jackshirazi]

Classic agent link

• https://www.elastic.co/guide/en/apm/agent/java/current/config-reporter.html#config-disable-send
• https://www.elastic.co/guide/en/apm/agent/java/current/config-core.html#config-context-propagation-only

What it does

Disable Send sets the agent to making no comms to APM server/collector, but everything else works (including creating transactions etc, eg for log correlation)
Context Propagation Only is the same as Disable Send except that the agent still connects to the APM-server for fetching configuration updates and health checks

Implementation difficulty, location, and dynamism

• Best done in the exporter (logs, metrics and traces exporters).
• It can be contributed upstream (but probably not worth it)
• Can be made dynamic but there is one gotcha, we need the resource bundle in case this is subsequently switched on

[jackshirazi]

Note this is done for the non-central config part in #474 ie the agent can now stop all sending dynamically. The central config client needs to support the ability to either stop checking (for the disable_send option) or continue checking (for context_propagation_only option) in the case where sending has been disabled

[AlexanderWert]

waiting on OpAMP implementation