[receiver/awscloudwatch] Use LogGroupIdentifier for cross-account log groups

[Vanshul97]

Summary

Fix cross-account log group discovery via AWS OAM by using LogGroupIdentifier (ARN) instead of LogGroupName in FilterLogEvents calls.

Fixes #46159

Problem

When using the AWS CloudWatch receiver with include_linked_accounts: true and account_identifiers for cross-account log collection via AWS OAM:

1. DescribeLogGroups correctly discovers log groups from linked accounts
2. But FilterLogEvents uses LogGroupName (just the name), which only searches the monitoring account's own log groups
3. AWS requires the full ARN via LogGroupIdentifier to read logs from linked accounts
4. Result: ResourceNotFoundException for every cross-account log group

Fix

• Added groupIdentifier field to streamNames and streamPrefix structs to store the log group ARN
• Modified discoverGroups() to capture lg.Arn from the DescribeLogGroups response
• Modified request() methods to use LogGroupIdentifier when an ARN is available, falling back to LogGroupName for single-account scenarios (backward compatible)

Test Plan

• All existing tests pass
• Code compiles cleanly
• Backward compatible — single-account scenarios continue using LogGroupName since ARN won't be set for non-cross-account groups

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