RUMM-2380: Enable custom application version support

[0xnm]

What does this PR do?

This change allows to set up custom app (package) version either through internal proxy API, or through using _dd.version in the additional config. Functionally it is the same as DataDog/dd-sdk-ios#973.

This change is needed to support OTA updates in the cross-platform frameworks like React Native where cross-platform code can be update without any application restart.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[codecov-commenter]

Codecov Report

Merging #1020 (ed486e6) into develop (b6e0e71) will increase coverage by 0.07%.
The diff coverage is 96.30%.

@@             Coverage Diff             @@
##           develop    #1020      +/-   ##
===========================================
+ Coverage    83.19%   83.26%   +0.07%     
===========================================
  Files          270      272       +2     
  Lines         9216     9235      +19     
  Branches      1469     1471       +2     
===========================================
+ Hits          7667     7689      +22     
+ Misses        1137     1136       -1     
+ Partials       412      410       -2     

Impacted Files	Coverage Δ
...oid/core/internal/system/NoOpAppVersionProvider.kt	66.67% <66.67%> (ø)
...n/com/datadog/android/core/internal/CoreFeature.kt	91.56% <85.71%> (+0.04%)	⬆️
...oid/src/main/kotlin/com/datadog/android/Datadog.kt	81.97% <100.00%> (+0.45%)	⬆️
.../main/kotlin/com/datadog/android/_InternalProxy.kt	87.50% <100.00%> (-1.39%)	⬇️
.../core/internal/system/DefaultAppVersionProvider.kt	100.00% <100.00%> (ø)
...adog/android/error/internal/CrashReportsFeature.kt	100.00% <100.00%> (ø)
.../src/main/kotlin/com/datadog/android/log/Logger.kt	95.97% <100.00%> (ø)
...atadog/android/log/internal/domain/LogGenerator.kt	98.97% <100.00%> (+0.02%)	⬆️
...otlin/com/datadog/android/rum/_RumInternalProxy.kt	100.00% <100.00%> (ø)
...lin/com/datadog/android/rum/internal/RumFeature.kt	93.00% <100.00%> (+0.07%)	⬆️
... and 13 more