This change implements the license-based enforcement of telemetry export as outlined in [the plan for the 5.2.1 release](https://docs.google.com/document/d/1Z1Yp7G61WYlQ1B4vO5-mIXVtmvzGmD7PqYHNBQV-2Ik/edit#bookmark=id.w10771x4dx7y).

Most of this functionality has been tested in various environments for some time: dotcom, s2, rctest, cody-dev, demo, and 9 trial Cloud instances (with more to be enabled before the final 5.2.1 release, see sourcegraph/customer#2416), processing about 2 million events in the last 7 days (a small set of events currently instrumented in the new system). The following changes are made to how this feature is enabled:

1. An export enablement mode is added to package `internal/licensing` that evaluates what we export based on the configured license, configured based on license plan or tags in the license, but the levels are unlikely to change):
   - all events (default going forward)
   - cody-only (`Feature=~'cody|cody.*'`), since Cody events are a special case covered by terms of use
   - nothing (air-gapped instance)
3. When persisting events to the export queue, the enablement mode is checked to decide if anything gets written to the database.
4. Exporter background workers are now enabled by default, checking the mode on each iteration to decide if we need to run the exporter at all. We also create a new connection to Telemetry Gateway on each run now, to ensure Telemetry Gateway downtime does not break instances.

The feature flag `telemetry-export` and env var `TELEMETRY_GATEWAY_EXPORTER_EXPORT_ADDR` remain configurable, but in a default-enabled state - in 5.2.2, the ability to disable these capabilities via configuration will be removed, and the license-based mechanism will become the only way to disable this feature, as requested in the [telemetry export rollout plan](https://docs.google.com/document/d/1Z1Yp7G61WYlQ1B4vO5-mIXVtmvzGmD7PqYHNBQV-2Ik/edit#bookmark=id.w10771x4dx7y). Parts that require follow-up changes are denoted with `TODO(5.2.2)` comments.

A changelog entry and docs are added in https://github.com/sourcegraph/sourcegraph/pull/57606

Part of https://github.com/sourcegraph/sourcegraph/issues/56816

- [x] New unit/integration test coverage
- [x] Existing tests pass with minimal changes
- [x] Manual testing:
  - Comment out `sg.config.yaml` line `TELEMETRY_GATEWAY_EXPORTER_EXPORT_ADDR: "http://127.0.0.1:10080"` and add `TELEMETRY_GATEWAY_EXPORTER_EXPORT_INTERVAL: "30s"`, exporter starts by default. Run some searches to generate events, and see that worker doesn't export anything, as the default mode for an older license is to disable export (seen in logs and metrics)
  - Set `dev-private` license to a new, real one from sourcegraph.com, remove all mentions of `SOURCEGRAPH_LICENSE_GENERATION_KEY` from `sg.config.yaml`, repeat above, events now export (seen in logs and metrics):
    `[         worker] INFO worker.telemetrygateway-exporter telemetrygatewayexporter/exporter.go:129 events exported {"maxBatchSize": 10000, "succeeded": 1}`

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Co-authored-by: Joe Chen <joe@sourcegraph.com>
(cherry picked from commit 20c771b)