auto generate `app_version` value

[keighrim]

This PR closes #114.

A few notes;

• app_version value is generated using git describe first, and then using $CLAMS_APP_VERSION environment variable, then falls back to unresolvable. git describe comes first to ensure the proper versions are inferred in case where multiple apps running all natively (not via containers). (d0006dc)
• The generation is done via pydantic default_factory, which won't prevent users from manually setting a value. I made it clearer in the documentation that this field (along with some others) is automatically filled, but existing apps need to be updated to "unset" app_versions. (no longer the case after f8c6965)
• The generated version number is then automatically appended to the identifier. It means all existing apps should also be updated if the identifier field includes the version number (like this). Also 3ee9829 allows using a "shortname" for identifier (see the comment below)
• In addition to these changes, a GHA workflow should be updated to set and pass $CLAMS_APP_VERSION at the container build time.

[codecov[bot]]

Codecov Report

Merging #118 (f8c6965) into develop (9266524) will increase coverage by 0.60%. The diff coverage is 94.11%.

@@             Coverage Diff             @@
##           develop     #118      +/-   ##
===========================================
+ Coverage    81.29%   81.89%   +0.60%     
===========================================
  Files            6        6              
  Lines          449      475      +26     
===========================================
+ Hits           365      389      +24     
- Misses          84       86       +2     

Flag	Coverage Δ
unittests	81.89% <94.11%> (+0.60%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
clams/appmetadata/__init__.py	91.54% <93.93%> (+0.32%)	:arrow_up:
clams/app/__init__.py	87.50% <100.00%> (-0.22%)	:arrow_down:

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[keighrim]

New commits added to

1. re-order app_version generation logic
2. completely disable manually setting app_version and mmif_version values
3. allow using "shortname" for identifier field (dev puts example-app -> automatically expanded to http://apps.clams.ai/example-app/v100)

[keighrim]

https://github.com/clamsproject/.github/commit/84c29a95b13d4b8537eef8d568fde6f978fa5f10 adds $CLAMS_APP_VERSION generation for container actions, but Containerfile/Dockerfiles in individual app repositories also need to be updated to have something like this.

ARG CLAMS_APP_VERSION
ENV CLAMS_APP_VERSION ${CLAMS_APP_VERSION}

(turned out that there's no way to pass a ENV value from docker build CLI, instead only ARG value is passable)