Added Sha3 as alternative

[wardbekker]

What this PR does / why we need it:

PR For discussion. Looking for a nice way to add Sha3_256 as a modern alternative to Sha2_256

Was looking how to I could implement template function syntax like:

Hash(salt, input) # -> defaulting to e.g. Sha2_256 
Hash(Sha3_256, salt, input) # -> explicit specification of hashing method 

But I couldn't find a way to implement function overloading in Go.

Hence, to don't go overboard on verbosity on the template function (e.g. Hash_Sha3_256(salt, input) ) I settled for Sha2(salt, input) and Sha3(salt, input).

I recommend to use only _256 result:

• good enough (TM) hashing for the intended use case.
• already a large number with 64 hex digits and high entropy numbers don't compress well, so _512 will add additional hex digits without any real benefit.

Checklist

• Documentation added
• Tests updated

[wardbekker]

added commit and added more context to PR description

[codecov-commenter]

Codecov Report

Merging #2480 into master will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2480      +/-   ##
==========================================
- Coverage   62.92%   62.90%   -0.03%     
==========================================
  Files         162      162              
  Lines       13981    13985       +4     
==========================================
- Hits         8798     8797       -1     
- Misses       4496     4498       +2     
- Partials      687      690       +3     

Impacted Files	Coverage Δ
pkg/logentry/stages/template.go	83.87% <100.00%> (+1.11%)	⬆️
pkg/promtail/targets/file/tailer.go	73.86% <0.00%> (-4.55%)	⬇️
pkg/promtail/targets/file/filetarget.go	67.85% <0.00%> (-1.79%)	⬇️
pkg/querier/queryrange/downstreamer.go	97.93% <0.00%> (+2.06%)	⬆️

[RichiH]

I think we should be precise and explicit, i.e. call the hash function SHA3-256 (or Sha3-256). At the same time, we are applying a hash function, so that should be stated.
To lessen typing load, I would come with opinionated defaults. That's why I still think the default should be

replace: '*SSN*{{ .Value | Hash }}*'

which then offers SHA3-256 & a configured hash.

[oddlittlebird]

Edited docs

[owen-d]

I think there's already been a lot of feedback here and I'm not opinionated enough/think bikeshedding further is worse than committing to something.

I like that we're exposing a Hash method designed to be kept strong over time. I dislike that we chose Sha2Hash over Sha256.

All in all I think this is a great PR I'm excited to have in Loki. Great work @wardbekker.

changes were addressed