Add GH actions for deploying example server to Fly

[ml-evs]

First PR addressing #1307 by deploying the app to Fly. Will also need a follow-up PR to remove all mentions of Heroku.

[codecov]

Codecov Report

Merging #1396 (7f63c15) into master (b21d894) will not change coverage.
The diff coverage is n/a.

❗ Current head 7f63c15 differs from pull request most recent head 5e93d39. Consider uploading reports for the commit 5e93d39 to get more accurate results

@@           Coverage Diff           @@
##           master    #1396   +/-   ##
=======================================
  Coverage   91.46%   91.46%           
=======================================
  Files          72       72           
  Lines        4369     4369           
=======================================
  Hits         3996     3996           
  Misses        373      373           

Flag	Coverage Δ
project	91.46% <ø> (ø)
validator	91.52% <ø> (ø)

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[ml-evs]

Seems to be working fine:

$ optimade-validator https://optimade.fly.dev
Testing entire implementation at https://optimade.fly.dev
.......✖.............................................................

OPTIONAL TEST FAILURES
======================

https://optimade.fly.dev/v123123 - _get_endpoint - failed with error
        ResponseError: Request to 'v123123' returned HTTP status code 404.
        Additional details from implementation:
          HTTPException: Not Found (N/A)


Passed 57 out of 57 tests.
Additionally passed 11 out of 12 optional tests.

This PR can't really be reviewed without testing, so I'll self-merge and then keep an eye on it.