Revamp Firely library

[ruimendesM]

Goal

On the new app we want to be able to integrate remote configs on the project.
Firely is a library done to help manage remote configs and A/B tests, however currently Firely library is completely outdated.
We want to bring it back to include these functionalities on the new app.

Description

• Convert current codebase to Kotlin
• Update outdated firebase dependencies
• Create a Circle CI pipeline for the library
  - [ ] (Bonus point) Investigate if it could make sense to have it in a Kotlin multiplatform library.

[filipebrandao]

Leaving a note for the future:

Firebase now allows to download a json file with all the Remote Configs keys and default values. Similar to firely-config.json but less structured. Maybe it would be nicer to just use the Firebase json file insted of having to recreate everything in our firely-config.json.

Only disadvantage is that we would loose access to the remote configs through :
FirelyConfig.FeatureFlag.FEATURE_FLAG_1, FirelyConfig.Config.SOME_CONFIG, FirelyConfig.Experiment.MY_EXPERIMENT
And instead it would be just:
FirelyConfig.FEATURE_FLAG_1, FirelyConfig.SOME_CONFIG, FirelyConfig.MY_EXPERIMENT

Basically, saves us work a bit of work but it's less structured.

[filipebrandao]

Some worries about making this lib multiplatform :

• It uses reflection and we have very limited capabilities in a multiplatform project
• It does code generation and even though the lib used in Chore/1 revamp #2 generated multiplatform kotlin code, the lib used to generate is not yet multiplatform itself Restructure the project for multiplatform Kotlin  square/kotlinpoet#304

[ruimendesM]

Task done and CI pipeline merged as part of #6 .
Next steps will be to publish both the gradle plugin (being done on #7) and the library later.

Based on the comments from @filipebrandao, we decided to discard the possibility of making this lib multiplatform at the moment