feat: Add support for job priority

[andoriyu]

Allows specifying priority when job is enqueued.

Closes #7

BREAKING CHANGE:

• SCHEMA_SQL is gone. MIGRATOR should be used instead. This means ADC must have exclusive rights to SQLx migrations table. Since it's rarely the case - use different schema/database for ADC.
• Interface to Queue for pushing jobs includes priority argument

[github-actions[bot]]

Unit Test Results

10 tests   10 :heavy_check_mark:  0s :stopwatch:   2 suites    0 :zzz:   2 files      0 :x:

Results for commit 4f0f970a.

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