ATO-2208: new version 2 signing keys build

[Louisasa]

Wider context of change

We want to rotating our external token signing keys again. As part of this we need to re-point our signing code at the new keys, so that we can use these new keys to sign our externally issued tokens. This adds a feature flag and enables it in dev/build.

What’s changed

• Adds a new feature flag to control which keys we use to sign our new tokens
• Conditionally signs our tokens using the new key depending on the flag

Manual testing

Checklist

• Lambdas have correct permissions for the resources they're accessing.
• Impact on orch and auth mutual dependencies has been checked.
• Changes have been made to contract tests or not required.
• Changes have been made to the simulator or not required.
• Changes have been made to stubs or not required.
• Successfully deployed to authdev or not required.
• Successfully run Authentication acceptance tests against sandpit or not required.

Related PRs

#7867

[sonarqubecloud]

Quality Gate failed

Failed conditions
78.8% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud