GenAI 0.9.0 + Common 0.8.0 release (#188)

Per [b/350789193](https://b.corp.google.com/issues/350789193) and
[b/350789851](https://b.corp.google.com/issues/350789851),

This prepares the release for GenAI `0.9.0` and Common `0.8.0`.

Version 0.8.0 (#177)

Bump version to 0.7.0 (#161)

Bump versions to 0.6.0 (generativeai) and 0.5.0 (common) (#145)

Bump release to 0.5.0 (genai) 0.4.0 (common) (#136)

Bump generative ai sdk to 0.4.0 (#124)

Bump release versions (#117)

Common and GenerativeAI SDKs are both getting a version bump

Encode default value for content (#82)

By default, kotlin serialization does not encode default values. In the case of content, we do want to serialize it.

release 0.2.1 (#72)

Co-authored-by: David Motsonashvili <[email protected]>

Release Version 0.2.0 (#58)

Co-authored-by: David Motsonashvili <[email protected]>