Investigate if it's possible to track SPM vs Cocoapods usage

[buenaflor]

Description

For internal stats it would be good if it's possible

[linear]

DART-302

[buenaflor]

Possible way (needs more investigation)

In Package.swift

.target(
    name: "sentry_flutter",
    dependencies: [...],
    swiftSettings: [
        .define("SENTRY_SPM")
    ]
)

In SentryFlutterPlugin.swift

#if SENTRY_SPM
let build = "spm"
#else
let build = "cocoapods"
#endif

# attach build to events