feat(splunk_hec source): support second-stage framing and decoding

changelog.d/splunk_hec_source_codec.enhancement.md

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+The `splunk_hec` source now accepts optional per-endpoint codec configuration via `event: { framing, decoding }` and `raw: { framing, decoding }`. When `decoding` is set on an endpoint, Vector applies a second decoding pass after the HEC envelope is parsed: on `/services/collector/event` the envelope's `event` field is fed through the codec (string contents are passed as raw bytes; objects, arrays, and other JSON values are JSON-serialized first to preserve shape), and on `/services/collector/raw` the request body is fed through the codec directly. A single payload can fan out to multiple events. Decoder-produced fields win on conflict with HEC envelope metadata, and decode failures do not surface to the Splunk client. Each endpoint is configured independently; when neither endpoint is configured the source preserves its existing behavior unchanged.
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+authors: thomasqueirozb