feature: healthcheck ready wait for all data sources load

[charlottebrady]

Fixes Issue

Closes #801

Changes proposed

This PR:
🌟 Adds a query to the /ready endpoint, defaults to false to maintain current behaviour.
🌟 Propagates the query through the system to the is_ready healthcheck method
🌟 If the query parameter = True, then we mark the system as 'ready' when the number of successful data transactions matches the total number of expected data transactions (defined from the opal data config sources)

• One issue I can see with this is that the /healthy endpoint should check the system is 'ready' (as defined by the ready endpoint) with a little extra too. However, I have not touched the /healthy code here.
• This should require a documentation update which I have not done yet.

Check List (Check all the applicable boxes)

• I sign off on contributing this submission to open-source
• My code follows the code style of this project.
• My change requires changes to the documentation.
• I have updated the documentation accordingly.
• All new and existing tests passed.
• This PR does not contain plagiarized content.
• The title of my pull request is a short description of the requested changes.

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