config: granular env-based solution for connection strings

[Samk13]

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Description

I rearranged the logic to prioritize individual environment variables (DB_HOST, DB_PASSWORD, etc.) to build URI.
If these are missing, it checks for the full value. As a fallback, it defaults to the final fallback value.
This asserts that user-provided values take precedence.
This PR includes:

• Build db URI
• Build Redis URL
• Build AMQP URL

Partially closes: inveniosoftware/helm-invenio#112

Questions:

• Should the logic remain in this package, or should it be moved to another one e.g.: invenio_app or invenio_config and imported here?

Checklist

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