Re-add docker blob build artifact type

[soapy1]

Fixes #1085

Description

This PR re-adds the DOCKER_BLOB build artifact enum value. This is required for backwards compatibility with old database entries.

To test manually:

• insert a new build into the database with the docker_blob artifact type
  one way to do this is to add a new artifact to an existing build. For example (with the docker compose setup):

$ docker compose exec -it psql
# \c conda-store

// see what build artifacts exist
conda-store=# select * from build_artifact;
 id | build_id | artifact_type |                                  key                                   
----+----------+---------------+------------------------------------------------------------------------
  1 |        1 | LOGS          | logs/c3457167-1739296499-1-python-flask-env.log
  2 |        1 | LOCKFILE      | lockfile/c3457167-1739296499-1-python-flask-env.yml
  3 |        2 | LOGS          | logs/d448494f-1739296648-2-test.log
  4 |        1 | DIRECTORY     | /var/lib/conda-store/filesystem/c3457167-1739296499-1-python-flask-env
  5 |        1 | YAML          | yaml/c3457167-1739296499-1-python-flask-env.yml
  6 |        2 | LOCKFILE      | lockfile/d448494f-1739296648-2-test.yml
  7 |        1 | CONDA_PACK    | archive/c3457167-1739296499-1-python-flask-env.tar.gz
  8 |        2 | DIRECTORY     | /var/lib/conda-store/test/d448494f-1739296648-2-test
  9 |        2 | YAML          | yaml/d448494f-1739296648-2-test.yml
 10 |        2 | CONDA_PACK    | archive/d448494f-1739296648-2-test.tar.gz

// add a docker blob to the build with id `2`
conda-store=# insert into build_artifact (build_id, artifact_type, key)  values (2, 'DOCKER_BLOB', 'idonte
xist');
INSERT 0 1

• query for the build using the api

$ curl http://localhost:8080/conda-store/api/v1/environment/

On the main branch this curl request causes an internal server error.

Pull request checklist

• Did you test this change locally?
• Did you update the documentation (if required)?
• Did you add/update relevant tests for this change (if required)?

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[peytondmurray]

Thanks!