change: enable stream proxy only by default

[spacewander]

Signed-off-by: spacewander spacewanderlzx@gmail.com

What this PR does / why we need it:

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[gxthrj]

It is confusing for using the configuration in stream-proxy to control the http-proxy.
It is better to keep configuration the same level with stream and http proxy.

[spacewander]

The HTTP proxy already at the top level of apisix. There can't be "the same level" unless we downgrade the level of HTTP proxy.

[oldthreefeng]

so, You need to declare only this is a global variable. otherwise confused .....