chore: add governing license and package metadata

[peter-matkovski]

Summary

• add the repository-level Stream Source Code License, copied byte-for-byte from stream-chat-js
• include the same license in the published stream-chat-angular package
• declare SEE LICENSE IN LICENSE in the package metadata
• link the contribution guidance to the local license file

Contributor-rights check

External contributions were accepted under this repository's mandatory Stream CLA policy. The policy was present when each merged external contribution was accepted. The CLA assigns contribution rights to Stream and applies retroactively. The remaining human contributors are Stream staff or former staff; automated release commits are attributable to Stream-controlled bots.

Validation

• both license copies match GetStream/stream-chat-js@master:LICENSE byte-for-byte
• package and ng-packagr configuration files parse as valid JSON
• ng-packagr is configured to copy LICENSE into the published package
• non-license changes pass git diff --check

Tracking: APPSEC-316