esi: Add VCL override for esi_include_onerror

[dridi]

Inspired by the discussion from #4053.

[dridi]

Discussion from #4053:

One thing I'm also wondering is whether to always initialize this from the parameter or do that only for req_top (technically, resp_top except that it does not exist) and then inherit the default value from req_top in sub-requests.

[nigoroll]

extra bugwash: after release, add vcl control over all esi feature flags