Minor refactor of proxy parsing to make upstreams re-usable

[johnbelamaric]

1. What does this pull request do?

Refactors upstream parsing so we can re-use it in other plugins than proxy. For example, fallback. Doing this in a separate PR means fallback can be external if we want.

It creates a function NewStaticUpstream that performs the parsing starting at the FROM.

2. Which issues (if any) are related?

#1382

3. Which documentation changes (if any) need to be made?

None.

[codecov-io]

Codecov Report

Merging #1426 into master will increase coverage by 0.02%.
The diff coverage is 94.44%.

@@            Coverage Diff            @@
##           master   #1426      +/-   ##
=========================================
+ Coverage   52.07%   52.1%   +0.02%     
=========================================
  Files         173     173              
  Lines        8465    8470       +5     
=========================================
+ Hits         4408    4413       +5     
  Misses       3718    3718              
  Partials      339     339

Impacted Files	Coverage Δ
plugin/proxy/upstream.go	83.2% <94.44%> (+0.7%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fcfb998...f24a67e. Read the comment docs.

[miekg]

I'm fine with this, but I'm not sure if you want proliferate proxy and maybe instead focus on forward
Also see #1443

[johnbelamaric]

Ok. Well, I will merge anyway, we can see about trying it with forward.