Add semantic conventions for http content size to the api/standard package

[MrAlias]

Attribute name	Notes and examples	Required?
http.request_content_length	The size of the request payload body in bytes. This is the number of bytes transferred excluding headers and is often, but not always, present as the [Content-Length][] header. For requests using transport encoding, this should be the compressed size.	No
http.request_content_length_uncompressed	The size of the uncompressed request payload body after transport decoding. Not set if transport encoding not used.	No
http.response_content_length	The size of the response payload body in bytes. This is the number of bytes transferred excluding headers and is often, but not always, present as the [Content-Length][] header. For requests using transport encoding, this should be the compressed size.	No
http.response_content_length_uncompressed	The size of the uncompressed response payload body after transport decoding. Not set if transport encoding not used.	No

Related to open-telemetry/opentelemetry-specification#641