RUMM-2381 Add internal proxy API for customising app version

[ncreated]

What and why?

📦 This PR adds set(customVersion:) API to internal proxy available in cross-platform SDKs. This is to unlock OTA use cases in RN, where version can change without restarting the app.

How?

I added AppVersionProvider, which reads initial version from configuration (supporting _dd.version override) and accepts its updates through _InternalProxy.configuration proxy.

Cross-platform SDKs can now customize version in 2 ways:

• on SDK init, by passing "_dd.version" in additionalConfiguration;
• later at runtime by using Datadog._internal._configuration.set(customVersion:).

Aware of architectural changes made in #947, notably the removal of FeatureCommonDependencies container, this new provider is added directly to DatadogCore - in the same way as done on V2 branch.

Review checklist

• Feature or bugfix MUST have appropriate tests (unit, integration)
• Make sure each commit and the PR mention the Issue number or JIRA reference
• Add CHANGELOG entry for user facing changes

Custom CI job configuration (optional)

• Run unit tests
• Run integration tests
• Run smoke tests

[maxep]

Perfect! It will be easy to migrate this to v2 👌