fix: prevent setting duplicate externalId types

About the issue

I observed an issue while setting the externalId's in identify event. Suppose when two identify events are made:

RudderAnalyticsUtils.analytics.identify(
    userId = "user 1",
    options = RudderOption().apply {
        putExternalId("brazeExternalID", "12345")
        putExternalId("amplitudeExternalID", "67890")
        putExternalId("adobeExternalID", "67890")
    }
)

RudderAnalyticsUtils.analytics.identify(
    userId = "user 1",
    options = RudderOption().apply {
        putExternalId("brazeExternalID", "12345-2")
        putExternalId("amplitudeExternalID", "67890-2")
        putExternalId("firebaseExternalID", "67890")
    }
)

Then the expected behaviour (as per v1 SDK) is to merge the externalId of both the identify events giving higher preference to the second identify event externalId. The final payload will look like this:

"externalId": [
    {
        "id": "12345-2",
        "type": "brazeExternalID"
    },
    {
        "id": "67890-2",
        "type": "amplitudeExternalID"
    },
    {
        "id": "67890",
        "type": "adobeExternalID"
    },
    {
        "id": "67890",
        "type": "firebaseExternalID"
    }
],

ISSUE: In v2, this wasn't the case, it was simply adding both identify externalIds values and no merging was taking place. So the externalId data was incorrect.

About the fix

We have solved the merging issues of the externalId's and added new test cases.

Type of change

[x] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[ ] This change requires a documentation update
Checklist:
[ ] Version upgraded (project, README, gradle, podspec etc)
[ ] My code follows the style guidelines of this project
[ ] I have performed a self-review of my own code
[ ] I have commented my code, particularly in hard-to-understand areas
[ ] I have added unit tests for the code
[ ] I have made corresponding changes to the documentation

rudderlabs / rudder-sdk-android