feat: Replace credentials-secret with db/broker connection(s)

[adwk67]

Description

Fixes #64

Notes for reviewers

• This is large PR although most of the files that have been changed are due to tests and docs: the main changes are in database.rs, queue.rs and connection.rs.
• We don't want the operator to have to read the secrets, and so the connection code (which will eventually be moved to the framework) returns a connection string containing embedded variables.
• These can be resolved in one of the following ways:
  • by shell variable substitution (if the variables are presented as normal env-vars e.g. $FOO) by processes started from the shell
  • by config-utils templating: this assumes that the variable is presented in the form of ${env:...} and has been written to a config file
  • N.B. templating of env-vars themselves was considered but rejected as (re-)writing env-vars from Rust using the env library is not safe in a multithreaded environment
• The operator is responsible for passing env-var names in the form that matches one of the above.
• Some connections contain a driver-dependent prefix, depending on the context. The operator makes the decision which type of connection string to retrieve (e.g. postgresql/sqlalchemyuri or postgresql/celery).
• Queues/brokers could be considered as being a subset of backends but have been kept separate as they were not specifically covered by ADR 29.

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

Author

• Changes are OpenShift compatible
• CRD changes approved
• CRD documentation for all fields, following the style guide.
• Helm chart can be installed and deployed operator works
• Integration tests passed (for non trivial changes)
• Changes need to be "offline" compatible
• Links to generated (nightly) docs added
• Release note snippet added

Reviewer

• Code contains useful comments
• Code contains useful logging statements
• (Integration-)Test cases added
• Documentation added or updated. Follows the style guide.
• Changelog updated
• Cargo.toml only contains references to git tags (not specific commits or branches)

Acceptance

• Feature Tracker has been updated
• Proper release label has been added
• Links to generated (nightly) docs added
• Release note snippet added
• Add type/deprecation label & add to the deprecation schedule
• Add type/experimental label & add to the experimental features tracker