feat: Add APM Server version fetching and client.supportsKeepingUnsampledTransaction()

[trentm]

This is to support the Node.js APM agent changing its behaviour based on the APM Server version. Specifically whether to send or drop unsampled transactions.

Refs: https://github.com/elastic/apm-agent-nodejs/issues/2455

[trentm]

The first commit here is a first stab that has the APM Server fetching retry if it fails on earlier attempts. IIUC the java agent does not retry (see https://github.com/elastic/apm-agent-java/blob/master/apm-agent-core/src/main/java/co/elastic/apm/agent/report/ApmServerHealthChecker.java). I think I am going to remove the retrying in the node.js agent, but I am not sure yet. If there are no retries, then if the APM Server happens to be down when the agent starts, then the agent will stay in a "degraded" state not knowing the APM server version. If this "degraded" state just assumes the latest APM Server version then that is perhaps fine, but it could mean not getting data to the (older) APM server version if there is a compat issue.

[apmmachine]

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#### Build stats * Reason: `null` * Start Time: 2022-01-25T23:33:24.385+0000 * Duration: 11 min 39 sec * Commit: fca8cf1063a481c8cb07caadb23dc9cf8b814ef9 #### Test stats :test_tube: | Test | Results | | ------------ | :-----------------------------: | | Failed | 0 | | Passed | 17206 | | Skipped | 0 | | Total | 17206 |

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[trentm]

elastic-apm-http-client@10.4.0 published v10.4.0 tagged