feat: added methods to store and retrieve state in the context store

[bflorian]

Added methods to the context object to get and set properties that are stored in the state field of the context record. These methods are useful for storing information associated with each installed app instance, such as third-party API access tokens, without implementing a separate data store.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] I have read the CONTRIBUTING document
• [x] My code follows the code style of this project (hint: install an xo editor plugin)
• [x] Any required documentation has been added
• [x] I have added tests to cover my changes

[smartthingspi]

:tada: This PR is included in version 4.3.0 :tada:

The release is available on:

• GitHub release
• npm package (@latest dist-tag)

Your semantic-release bot :package::rocket: