[chore] [exporter/splunkhec] Simplify the batching process

[dmitryax]

Rework how the batches are created and sent to the backend to make it easier to read the code.

Also, this change fixes an issue introduced by removing the additional index field of the bufferState struct in #21765: if a hec batch combining several OTel resources gets rejected with a retriable error, only the latest read resource/scope records were sent for retry. The fix is confirmed by an additional TestPushLogsRetryableFailureMultipleResources test.

This also significantly improves performance by removing a bunch of memory allocations in cases where the incoming data ends up being sent in several HEC batches.

Before:

goos: darwin
goarch: arm64
pkg: github.com/open-telemetry/opentelemetry-collector-contrib/exporter/splunkhecexporter
Benchmark_pushLogData_10_10_1024
Benchmark_pushLogData_10_10_1024-10    	   12033	     99060 ns/op	   90027 B/op	    1299 allocs/op
Benchmark_pushLogData_10_10_8K
Benchmark_pushLogData_10_10_8K-10      	   12072	     96328 ns/op	   90065 B/op	    1299 allocs/op
Benchmark_pushLogData_10_10_2M
Benchmark_pushLogData_10_10_2M-10      	   12306	     97606 ns/op	   90114 B/op	    1299 allocs/op
Benchmark_pushLogData_10_200_2M
Benchmark_pushLogData_10_200_2M-10     	     572	   2104811 ns/op	 1828672 B/op	   26003 allocs/op
Benchmark_pushLogData_100_200_2M
Benchmark_pushLogData_100_200_2M-10    	      56	  19257562 ns/op	18350930 B/op	  259916 allocs/op
Benchmark_pushLogData_100_200_5M
Benchmark_pushLogData_100_200_5M-10    	      55	  19130223 ns/op	18467195 B/op	  259916 allocs/op
PASS

After:

Benchmark_pushLogData_10_10_1024
pkg: github.com/open-telemetry/opentelemetry-collector-contrib/exporter/splunkhecexporter
Benchmark_pushLogData_10_10_1024
Benchmark_pushLogData_10_10_1024-10    	   34998	     34225 ns/op	   30894 B/op	     450 allocs/op
Benchmark_pushLogData_10_10_8K
Benchmark_pushLogData_10_10_8K-10      	   12380	     94922 ns/op	   87420 B/op	    1264 allocs/op
Benchmark_pushLogData_10_10_2M
Benchmark_pushLogData_10_10_2M-10      	   12411	     96700 ns/op	   90155 B/op	    1299 allocs/op
Benchmark_pushLogData_10_200_2M
Benchmark_pushLogData_10_200_2M-10     	     560	   2088985 ns/op	 1827450 B/op	   26003 allocs/op
Benchmark_pushLogData_100_200_2M
Benchmark_pushLogData_100_200_2M-10    	      70	  15065196 ns/op	14020531 B/op	  198538 allocs/op
Benchmark_pushLogData_100_200_5M
Benchmark_pushLogData_100_200_5M-10    	      55	  18660002 ns/op	18467047 B/op	  259916 allocs/op
PASS