[wasm] Trailer headers not implemented

[pavelsavara]

As covered by unit test SocketsHttpHandler_Http1_TrailingHeaders_Test and https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Trailer

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Issue Details

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As covered by unit test SocketsHttpHandler_Http1_TrailingHeaders_Test and https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Trailer

Author:	pavelsavara
Assignees:	-
Labels:	arch-wasm, area-System.Net.Http
Milestone:	-