Change implementation to api in build.gradle.kts so subdependencies are exposed

[edwardmp]

For now I changed all of them to use api, even though for my own use case I don't need all of them to be exposed, but I suspect sooner or later someone else will want to use classes in any of these modules anyway.
Thoughts?

Fixes #54

[edwardmp]

CC @lilgallon

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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[lilgallon]

Thanks for your contribution, I am not sure about exposing ocpp-version specific modules. I will talk with my team about that

[edwardmp]

@lilgallon I'm actually quite sure you need it.
For instance, in the example in the linked issue, you need the com.izivia.ocpp.core16.ChargePointOperations class for the api callbacks, so the ocpp-1-6-core needs to definitely be exposed (same for other verisons)

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud