fix: align SDK metadata field names with Android and JS SDKs

[leoromanovsky]

Motivation

iOS SDK was using different metadata field names (sdkName, sdkVersion) compared to Android and JS SDKs (sdkLanguage, sdkLibVersion). This inconsistency can cause issues with downstream data processing.

Changes

• Renamed metadata key sdkName → sdkLanguage in EppoClient.swift
• Renamed metadata key sdkVersion → sdkLibVersion in EppoClient.swift
• Applied same changes to EppoPrecomputedClient.swift

Decisions

• Only changed the metadata dictionary keys that are sent over the wire
• Internal variable names remain unchanged (sdkName, sdkVersion constants)
• Aligned with Android SDK (EppoPrecomputedClient.java:529-530) and JS SDK (eppo-client.ts:1342-1343)

[sameerank]

Would be nice to also match up the internal variable names across the code

eppo-ios-sdk/Sources/eppo/Constants.swift

Lines 2 to 3 in 0e4f22b

	public let sdkName = "ios"
	public let sdkVersion = "5.3.1"

But this is the minimal change that is sufficient for now!

[leoromanovsky]

Would be nice to also match up the internal variable names across the code

eppo-ios-sdk/Sources/eppo/Constants.swift

Lines 2 to 3 in 0e4f22b

	public let sdkName = "ios"
	public let sdkVersion = "5.3.1"

But this is the minimal change that is sufficient for now!

claude wanted to do that too but i reigned it in for minimal diff 😄