fix: data race in convert_utf8_bytes plugin

[DmitryRomanov]

Description

The plugin reused a single p.buf buffer across events, causing a race
when concurrent processors modified it while outputs were encoding.
Fixed by using per-event event.Buf instead of plugin-shared p.buf.

WARNING: DATA RACE
Read at 0x00c00512a000 by goroutine 72:
  github.com/ozontech/insane-json.escapeString()
      github.com/ozontech/insane-json@v0.1.9/insane.go:2026 +0x3f2
  github.com/ozontech/insane-json.(*Node).Encode()
      github.com/ozontech/insane-json@v0.1.9/insane.go:660 +0x7ee
  github.com/ozontech/file.d/pipeline.(*Event).Encode()
      github.com/ozontech/file.d/pipeline/event.go:167 +0x75
  github.com/ozontech/file.d/encoder.(*JSONEncoder).Encode()
      github.com/ozontech/file.d/encoder/json.go:14 +0x3b
  github.com/ozontech/file.d/plugin/output/http.(*Plugin).out.func1()
      github.com/ozontech/file.d/plugin/output/http/http.go:392 +0x6a5
  github.com/ozontech/file.d/pipeline.(*Batch).ForEach()
      github.com/ozontech/file.d/pipeline/batch.go:106 +0x980
  github.com/ozontech/file.d/plugin/output/http.(*Plugin).out()
      github.com/ozontech/file.d/plugin/output/http/http.go:388 +0x473
  github.com/ozontech/file.d/plugin/output/http.(*Plugin).out-fm()
      <autogenerated>:1 +0x3e
  github.com/ozontech/file.d/pipeline.(*RetriableBatcher).Out()
      github.com/ozontech/file.d/pipeline/backoff.go:58 +0x248
  github.com/ozontech/file.d/pipeline.(*RetriableBatcher).Out-fm()
      <autogenerated>:1 +0x3e
  github.com/ozontech/file.d/pipeline.(*Batcher).work()
      github.com/ozontech/file.d/pipeline/batch.go:207 +0x268
  github.com/ozontech/file.d/pipeline.(*Batcher).Start.gowrap1()
      github.com/ozontech/file.d/pipeline/batch.go:189 +0x2e

Previous write at 0x00c00512a000 by goroutine 74:
  runtime.slicecopy()
      runtime/slice.go:392 +0x0
  github.com/ozontech/file.d/plugin/action/convert_utf8_bytes.(*Plugin).convert()
      github.com/ozontech/file.d/plugin/action/convert_utf8_bytes/convert_utf8_bytes.go:170 +0x157
  github.com/ozontech/file.d/plugin/action/convert_utf8_bytes.(*Plugin).Do()
      github.com/ozontech/file.d/plugin/action/convert_utf8_bytes/convert_utf8_bytes.go:155 +0x14b
  github.com/ozontech/file.d/pipeline.(*processor).doActions()
      github.com/ozontech/file.d/pipeline/processor.go:206 +0x236
  github.com/ozontech/file.d/pipeline.(*processor).processEvent()
      github.com/ozontech/file.d/pipeline/processor.go:170 +0x6b
  github.com/ozontech/file.d/pipeline.(*processor).processSequence()
      github.com/ozontech/file.d/pipeline/processor.go:149 +0x34
  github.com/ozontech/file.d/pipeline.(*processor).dischargeStream()
      github.com/ozontech/file.d/pipeline/processor.go:141 +0x44
  github.com/ozontech/file.d/pipeline.(*processor).process()
      github.com/ozontech/file.d/pipeline/processor.go:129 +0x64
  github.com/ozontech/file.d/pipeline.(*processor).start.gowrap1()
      github.com/ozontech/file.d/pipeline/processor.go:118 +0x2e