fix(#137): use nullable boolean type for the Release boolean fields

[prizov]

Why

The changes fix an issue when Helmsman doesn't overwrite positive boolean values with negatives for Helm releases while merging multiple DSFs(#137).

What

• Adding a new type NullBool (inspired by NullBool tyoe from sql package)
• Adding a mergo transormer with custom merging logic(overwriting) for the new type
• Changing the Release bool fields to the NullBool type
• Adding corresponding tests
• Adjusting the JSON schema. The NullBool type is considered as boolean type, so there are no changes from the usage perspective

Verification

make test + verified manually

Example

charts/common.yaml

helmRepos:
  jenkins: https://charts.jenkins.io

namespaces:
  staging:

apps:
  jenkins:
    namespace: staging
    enabled: true
    chart: jenkins/jenkins
    version: 2.15.1

charts/custom.yaml

apps:
  jenkins:
    enabled: false

charts/spec.yaml

stateFiles:
  - path: ./common.yaml
  - path: ./custom.yaml

Expected behavior

The jenkins app has enabled field set to false and accordingly Helmsman shouldn't install this release

Current behavior

➜ helmsman --debug --spec spec.yaml
...
Applications: 
--------------- 

        name:  jenkins
        description:  
        namespace:  staging
        enabled:  true
        chart:  jenkins/jenkins
        version:  2.15.1
...
2022-08-31 19:48:06 NOTICE: -------- PLAN starts here --------------
2022-08-31 19:48:06 NOTICE: Release [ jenkins ] in namespace [ staging ] will be installed using version [ 2.15.1 ] -- priority: 0
2022-08-31 19:48:06 NOTICE: -------- PLAN ends here --------------

With proposed changes

➜ helmsman --debug --spec spec.yaml
...
Applications: 
--------------- 

        name:  jenkins
        description:  
        namespace:  staging
        enabled:  false
        chart:  jenkins/jenkins
...
2022-08-31 19:49:03 NOTICE: -------- PLAN starts here --------------
2022-08-31 19:49:03 INFO: Release [ jenkins ] in namespace [ staging ]is disabled -- priority: 0
2022-08-31 19:49:03 NOTICE: -------- PLAN ends here --------------

[luisdavim]

🥇 Thanks