Motivation:
`HttpConversionUtil#toFullHttpResponse` and `HttpConversionUtil#toFullHttpRequest` has `ByteBufAllocator` which is used for building `FullHttpMessage` and then data can be appended with `FullHttpMessage#content`. However, there can be cases when we already have `ByteBuf` ready with data. So we need a parameter to add `ByteBuf` directly into `FullHttpMessage` while creating it.
Modification:
Added `ByteBuf` parameter,
Result:
More functionality for handling `FullHttpMessage` content.
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