Add 'splunk_hec' exporter, 'filter' processor, and 'transform' processor

[mkocher]

These are widely useful exporters we've gotten requests to include. Please note that like most things in otel-collector these are not considered stable upstream and the functionality and configuration may change over time as the otel-collector changes.

• splunk_hec (beta): Useful for sending logs & metrics to Splunk Enterprise. Eventually we expect this to replace the widely used splunk-firehose-nozzle
• filter (alpha): useful for dropping logs that are not worth the cost of sending to a particular destination
• transform (alpha): useful for modifying data in flight, like dropping or adding attributes