Add support for '/_fields' endpoint

[lukasmalkmus]

This has long been outstanding but went unnoticed: There has been a new /_info endpoint for a while, now. This is basically a list of dataset infos as returned by a normal call to :id/info. The /_stats call shares the same response model, apart from omitting the fields so this was never an issue. But it is desirable to have the separate call available.

I was able to refactor this slightly to compensate for the missing fields and enhanced the documentation along the way.

As explained by @mhr3, the /_info endpoint is deprecated and - after discussion - will be replaced by /_fields. The rest of the information that could be obtained through /_info is still available through /_stats. The recent commit accounts for that change but I kept the improvements to the DatasetInfo and DatasetStats types as well as the documentation improvements. Until the change is live, this PR will remain in draft status.

After more evaluation, there will be one new endpoint introduced with this PR: /_fields, for retrieving fields of all datasets of a deployment. However, in addition, /_info is deleted as it is essentially the infomration provided by /_fields plus some information only useful for the frontend but not actual costumers. The recent commit accounts for that change but I kept the improvements to the DatasetInfo and DatasetStats types as well as the documentation improvements. Until the change is live, this PR will remain in draft status.

Needs #106.

[mhr3]

We'll drop the /_infos endpoint, so let's not add it here

[codecov]

Codecov Report

Merging #104 (6e67a49) into main (e77ec89) will increase coverage by 0.06%.
The diff coverage is 81.81%.

❗ Current head 6e67a49 differs from pull request most recent head 40863e8. Consider uploading reports for the commit 40863e8 to get more accurate results

@@            Coverage Diff             @@
##             main     #104      +/-   ##
==========================================
+ Coverage   71.85%   71.91%   +0.06%     
==========================================
  Files          41       41              
  Lines        1304     1314      +10     
==========================================
+ Hits          937      945       +8     
- Misses        222      223       +1     
- Partials      145      146       +1     

Impacted Files	Coverage Δ
axiom/apl/format.go	100.00% <ø> (ø)
axiom/monitors.go	68.62% <ø> (ø)
axiom/notifiers.go	64.10% <ø> (ø)
axiom/orgs.go	66.19% <ø> (ø)
axiom/query/aggregation.go	84.21% <ø> (ø)
axiom/query/filter.go	88.00% <ø> (ø)
axiom/query/kind.go	70.00% <ø> (ø)
axiom/query/result.go	72.00% <ø> (ø)
axiom/starred.go	58.33% <ø> (ø)
axiom/tokens.go	76.66% <ø> (ø)
... and 3 more

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[lukasmalkmus]

This is ready for review and can be merged when CI passes on staging.

[lukasmalkmus]

Updated to PR description to reflect the latest state of the discussion.

I re-ran tests and made some minor adjustments.

[github-actions]

This PR/issue depends on:

• Support Go 1.18, phase out Go 1.16 #106
  By Dependent Issues (🤖). Happy coding!

[lukasmalkmus]

This is finally ready to merge!

[bahlo]

LGTM