RUM-1053 Make Logger Sendable for Swift 6 concurrency

What and why?

📦 This PR prepares DatadogLogs for compatibility with apps that enable "complete" concurrency checking in Swift 6.

I identified a potential data-race safety issue when passing instances of LoggerProtocol across isolation boundaries, such as to different threads:

import DatadogLogs

let logger = Logger.create(with: .init())

DispatchQueue.global(qos: .userInitiated).async {
    logger.debug("foo") 
    // ^ ❌ Error: Capture of 'logger' with non-sendable type 'any LoggerProtocol' in a `@Sendable` closure
}

How?

This issue is resolved by declaring Sendable conformance for LoggerProtocol:

A conformance to Sendable means the given type is thread safe, and values of the type can be shared across arbitrary isolation domains without introducing a risk of data races.

Since Xcode 15 uses SWIFT_STRICT_CONCURRENCY: minimal, which checks only explicit Sendable conformances, this:

- public protocol LoggerProtocol {
+ public protocol LoggerProtocol: Sendable {

resulted in concurrency warnings elsewhere.

Instead of using @unchecked Sendable for RemoteLogger, ConsoleLogger, and some test types, I chose to address concurrency warnings properly, making RemoteLogger and ConsoleLogger fully checked Sendable types 👌.

This required several major changes:

Followed https://github.com/DataDog/dd-sdk-ios/pull/1744 and replaced the mutable core reference in RemoteLogger with an immutable reference to FeatureScope.
Made FeatureScope sendable, to facilitate similar work in other modules.
Reworked how global Logs attributes are passed to RemoteLogger after removing the core reference.
Introduced SynchronizedAttributes and SynchronizedTags to ensure RemoteLogger has only Sendable members.
Added test coverage for Logger tags, which was previously missing.

Smaller adjustments were made as a consequence of spreading Sendable conformance. For example, marking BacktraceReporter as Sendable required resolving issues in Crash Reporting tests, as this type is shared through DatadogInternal.

Review checklist

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

DataDog / dd-sdk-ios