add event exporter helper

[fspmarshall]

Implements an exporter helper type intended to make it easier to work with the new bulk event export API introduced in #46399. Fundamentally, this type is basically just doing a fancy version of:

for chunk := range getChunks(date) {
    for event := range getEvents(chunk) {
        export(event)
    }
}

Special care has been taken to ensure that per-event processing and cursor state tracking is simple and performant. Logging has also had custom rate limits applied to help mitigate issues we've seen in testing where event export logs can easily become overwhelming due to the high throughput in large clusters.

This is part of an ongoing body of work related to #46193

[public-teleport-github-review-bot]

@fspmarshall See the table below for backport results.

Branch	Result
branch/v14	Failed
branch/v15	Failed
branch/v16	Failed