feat: support custom resolve base URL

[nicklasl]

Summary

• add a resolveBaseUrl factory option for resolve and apply requests
• preserve regional defaults and keep event tracking on the Confidence events endpoint
• document sidecar setup

Testing

• focused resolve base URL tests
• existing remote client tests
• assembleDebug
• ktlintCheck

[nicklasl]

Verified this end to end with ConfidenceDemoApp on a Pixel 9 emulator running Android 16.

Setup:

• Ran a deterministic local HTTP resolver on localhost:8090.
• Used adb reverse tcp:8090 tcp:8090 so emulator localhost reached the host server.
• Temporarily enabled cleartext HTTP and configured the demo with http://localhost:8090. Both changes were restored afterward.
• Built :ConfidenceDemoApp:assembleDebug, installed it, cleared app data, and cold-started it.

SDK flow exercised:

ConfidenceFactory.create(
    context,
    clientSecret,
    "http://localhost:8090",
    initialContext = initialContext,
    region = ConfidenceRegion.EUROPE,
    loggingLevel = LoggingLevel.VERBOSE
)
ConfidenceFeatureProvider.create(confidence, InitialisationStrategy.FetchAndActivate)
OpenFeatureAPI.setProviderAndWait(provider)
client.getStringValue("flaggy.str", "client side default")
client.getStringDetails("hawkflag.color", "Gray")

Verification:

• Captured POST /v1/flags:resolve at the configured base URL.
• The mock response returned flaggy.str = custom-base-url-e2e and hawkflag.color = magenta with resolveToken = e2e-token.
• Captured two POST /v1/flags:apply requests at the same base URL, both carrying e2e-token.
• SDK logs showed Resolve Completed and TARGETING_MATCH with the expected variant and no error.
• The running app displayed Hello custom-base-url-e2e! and rendered the surface magenta.

No demo-app or E2E scaffolding changes were committed; the worktree was clean after verification.