Send webhooks via the Agent EVP Proxy when supported

[albertvaka]

What does this PR do?

• Checks whether the Agent supports the EVP Proxy endpoint.
• If it does, it generates webhooks instead of traces and sends them using the EVP Proxy.
• If it doesn't, continues to generate traces and send them via APM.

I also added tests for the whole webhooks logic, which didn't have any. They are mostly copies of the tests for the traces logic.

Description of the Change

• Adds a fetchAgentSupportedEndpoints() method to DatadogAgentClient that queries the /info Agent API.
• Makes the postWebhook() method part of the abstract DatadogClient so that DatadogWebhookBuildLogic and DatadogWebhookPipelineLogic can call it on either a DatadogAgentClient or a DatadogHttpClient instance.
• Implements postWebhook() in DatadogAgentClient. It sends webhook-type payloads using the Agent EVP Proxy.
• The public methods in DatadogTraceBuildLogic and DatadogWebhookBuildLogic are now @Overrides of abstract methods of their parent DatadogBaseBuildLogic so DatadogAgentClient can use either implementation (depending on the output of fetchAgentSupportedEndpoints). Same for Datadog*PipelineLogic.

Verification Process

• Test against an Agent that supports EVP Proxy and one that doesn't

Open Questions

• If Jenkins starts before the Agent, then the EVP Proxy support will be disabled because we won't be able to query the /info API when initializing the DatadogAgentClient. Is this okay?
  • ~I've added 3 retries at startup time to wait for the Agent.~
  • I've made checking the /info endpoint lazy. See my comment below.

[albertvaka]

I've changed the detection of the EVP Proxy support to be lazy, so it is checked when it's first needed and the result is cached for 1 hour.

This way we don't block at boot time and in the worst case scenario we add a 10 second delay (the timeout of the /info request) once every hour. Most times, the Agent will respond in a few milliseconds or not respond at all (ie: if it's not there or doesn't support the /info API) which will not block.

This also lets us support Agent upgrades without having to restart Jenkins.