Issue #76: Add configurable update rate

[vililahtevanoja-solibri]

Issue #, if available: #76

Description of changes:

• Add configurable update interval and back-off time. Slight refactoring was required in order to transfer the values from the GitHub inputs to where they are used. SDK-specific configuration parameters and general action configuration values (currently just the update rate configuration) are treated slightly differently and are passed as separate objects to the building and waiting functions.
• Add new test for handling the update rate configuration
• Update existing tests where needed
• Add new inputs to action metadata file
• Update README.md

Default values are kept the same (30s interval, 15s back-off), so there should be no breakage when updating.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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• [ ] Were any files moved? Moving files changes their URL, which breaks all hyperlinks to the files.

[vililahtevanoja]

This can be closed. It is superseded by #116.