Netty echo server support of h2 content aggregation for h2c upgrade scenario

[ldclakmal]

Description:
The EchoHttp2ServerHandler of Netty echo server, echo backs the header/data frames as soon as they arrive without any content aggregation against stream id. But content aggregation is needed because of [1]. So, h2 content aggregation is implemented for h2 scenario with the use of InboundHttp2ToHttpAdapter [2].

Need to add Netty echo server support of h2 content aggregation for h2c upgrade scenario as well in order to fix [3] [4].

[1] ballerina-platform/ballerina-lang#13953
[2] #23
[3] ballerina-platform/ballerina-lang#13927
[4] ballerina-platform/ballerina-lang#13954