Consistently declare Protobuf enums

[tigrannajaryan]

Enums were not consistently declared. Some were at the global scope,
some others wire nested in messages. This moves all enums to the global
scope and uses most of the official style guide rules here:
https://developers.google.com/protocol-buffers/docs/style#enums

Unfortunately this results in long and stuttering names of enum values in
the code but this change is necessary to future-proof the proto and ensure
no enum names conflict when we add new enums and values (all enum value
names are in one namespace).

I don't like the end result but not doing it appears to be worse.
We have seen this first-hand in OTLP proto where enum names are
inconsistent and are now impossible to fix due to backward compatibility
concerns.

Note: I ignored the rule to CAPITALIZE names and use underscores
because if I do that the names in the code become unreadable walls
of text.

If anyone has any ideas how to fix the enum names issue in a nicer
way I am happy to discard this PR.

One alternate is to remove the prefixes from enum value names and
just require that all enum value names introduced in the future are
also unique, but I think it can result in weird names in the future.

[codecov]

Codecov Report

Merging #115 (57922c7) into main (89eeb30) will increase coverage by 2.11%.
The diff coverage is 82.60%.

❗ Current head 57922c7 differs from pull request most recent head 3e0ac5c. Consider uploading reports for the commit 3e0ac5c to get more accurate results

@@            Coverage Diff             @@
##             main     #115      +/-   ##
==========================================
+ Coverage   74.61%   76.72%   +2.11%     
==========================================
  Files          23       23              
  Lines        1765     1706      -59     
==========================================
- Hits         1317     1309       -8     
+ Misses        334      298      -36     
+ Partials      114       99      -15     

Impacted Files	Coverage Δ
client/internal/packagessyncer.go	59.25% <60.00%> (ø)
client/internal/clientcommon.go	81.00% <77.77%> (+1.19%)	⬆️
client/internal/inmempackagestore.go	76.47% <100.00%> (ø)
client/internal/receivedprocessor.go	84.28% <100.00%> (+15.00%)	⬆️
client/httpclient.go	94.73% <0.00%> (ø)
client/internal/httpsender.go	72.26% <0.00%> (+1.29%)	⬆️
server/serverimpl.go	60.65% <0.00%> (+2.28%)	⬆️

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[tigrannajaryan]

Rebased and fixed merge conflicts. @andykellr if you have time please take another look (just want to make sure I didn't miss anything), otherwise I will merge soon.