Add support for DatabaseSessionService

[myakhnis-siro]

Hello! When will the DatabaseSessionService described here in the docs be available for adk-js? We can of course implement our own, but was curious about timing.

[kalenkevich]

Hello, you are right in time!
WIP: #129

[myakhnis-siro]

Hi, are there any updates on timing and/or will this be included in the next release? This will be a huge step towards production-readiness - excited to use it!

[kalenkevich]

@google/adk v0.4.0 just release, database session service is now available

[titaniumcloudwalk]

@kalenkevich Can documentation be updated to reflect that release?