mrc-6097 Metadata endpoint

[EmmaLRussell]

This branch implements a basic endpoint to provide metadata about the discovered datasets. For the test datasets the data part of the response looks like:

{
  "datasets": {
    "tile": {
      "gadm41": {
        "levels": [
          "admin0",
          "admin1"
        ]
      }
    }
  }
}

We currently only support one dataset type, "tile", with the idea that other datasets may be supported later.

When this happens we might change the metadata format somewhat, e.g. maybe a dataset supports multiple types of data, with shared feature ids. To be determined...

The static metadata is built on server start-up, from the discovered datasets, in a new buildMetadata method.

Other changes in this branch:

• some of the auxiliary stuff used by server, including buildMetadata, moved to a server folder to avoid top level getting too cluttered. server.ts itself it still at the top level - I'll move that if it's too confusing!
• Helper method in integrationTest: getData which expects a JSON success response and returns body.data.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.41%. Comparing base (b4c775a) to head (be540b4).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #4      +/-   ##
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+ Coverage   98.27%   98.41%   +0.13%     
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  Files          13       15       +2     
  Lines         116      126      +10     
  Branches       15       15              
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+ Hits          114      124      +10     
  Misses          2        2              

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[absternator]

LGTM!!! just a capitalization comment