[SPIKE] External components

[alexluckett]

Proposed change

POC to explore support for components where the input is external to the form journey. Example use-cases:

• Postcode lookup for UK addresses
• Maps
• GOV.UK Pay

This PR is a draft to explore an implementation. It may not reflect the final feature.

Jira ticket: DF-537

Process

• External component is built and pushed into the codebase. External components are standard components that expose a getRoutes() method and will usually have a consistent formSchema and stateSchema.
• On server startup, we analyse all components registered with the plugin. Any components exposing getRoutes() will have their routes collected and added to the hapi server.
• An external component is added to a form. Component is rendered as normal from its component class and associated view. The external component displays a secondary button prompting the user to take the mini journey. This button is a submit button.
• User completes the mini journey
• The external component sets yar.flash('externalStateAppendage', data) where data is a JSON string containing { component: 'name-of-component', data: { ... } }. Note this was previously done with redirects ?component=""&data="{}" but I removed it to avoid manipulation by a user.
• When preparing the form context, the engine checks for the presence of externalStateAppendage. It is parsed as a JSON string, the content is validated against the component's isState method. If successful, it is merged into the main form state.

Type of change

• Bug fix
• New feature
• Breaking change
• Misc. (documentation, build updates, etc)

Checklist

• You have executed this code locally and it performs as expected.
• You have added tests to verify your code works.
• You have added code comments and JSDoc, where appropriate.
• There is no commented-out code.
• You have added developer docs in README.md and docs/* (where appropriate, e.g. new features).
• The tests are passing (npm run test).
• The linting checks are passing (npm run lint).
• The code has been formatted (npm run format).

[sonarqubecloud]

Quality Gate failed

Failed conditions
4 Security Hotspots
0.0% Coverage on New Code (required ≥ 90%)
5 New Major Issues (required ≤ 0)

See analysis details on SonarQube Cloud

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[alexluckett]

Feature delivered under #220.