Migrate feature-to-feature messaging to typed MessageBus

[maxep]

⚠️ This PR is not ready for review and will be split into smaller PRs before merging.

What and why?

The legacy feature bus was a broadcast mechanism: every message sent on the bus was delivered
to all registered features, each of which had to pattern-match to determine whether the message
was relevant to it. Message payloads were typed as Any, making contracts implicit — senders
and receivers relied on shared knowledge of the underlying type with no compile-time enforcement.

This PR introduces a typed, subscription-based message bus. Each message is an explicit Swift
type conforming to BusMessage. Features declare interest in specific message types by subscribing
a BusMessageReceiver<Message> to the MessageBus. The bus routes each message only to
subscribers that declared interest in it, eliminating broadcast dispatch and making message
contracts explicit and compiler-verified.

How?

BusMessage is a protocol with a static var key: String used as the routing key.
BusMessageReceiver<Message: BusMessage> is AnyObject-constrained so receivers can be
identified by ObjectIdentifier for subscription and unsubscription. MessageBus exposes
typed subscribe(receiver:) and subscribe(block:) (returning an opaque
MessageBusSubscription handle) alongside a typed send<Message>(_ message:).
CoreMessageBus implements this in DatadogCore, dispatching each message only to
subscribers registered for that key.

Features subscribe their receivers to the bus at their convenience.

Migrated messages:

• CrashMessage, LogMessage (crash reporting ↔ logs)
• RUMErrorMessage, RUMFlagEvaluationMessage (feature flags → RUM)
• TelemetryMessage (all features → RUM), merging TelemetryInterceptor into TelemetryReceiver
• Crash-context payloads: RUMViewEvent, RUMViewReset, RUMSessionState, RUMEventAttributes, LogEventAttributes
• DatadogContext propagation (full opt-in subscription model)
• WebViewLogMessage, WebViewRUMMessage, WebViewRecordMessage

FeatureMessage, FeatureMessageReceiver, and MessageSending are now deprecated.
One intentional holdout: ProfilerStop payload (profiling still in development).

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
• Add Objective-C interface for public APIs - see our guidelines (internal)
• Run make api-surface when adding new APIs