Continuous integration, version constraints

[ryancco]

• Fixed an issue in src/Offers/OffersListDto.php causing psalm to fail - began failing after a hotfix for Typed property Affise\Sdk\Offers\OffersListDto::$notes must be string, null used #1 released in v0.1.1
• Added a minimum version constraint for guzzlehttp/psr7:^1.7.0 - supports usage of, for example, GuzzleHttp\Psr7\Header in src/AbstractTransport
• Add a minimum version constraint for guzzlehttp/guzzle:^6.5.2 - supports usage of, for example, GuzzleHttp\Handler\MockHandler::createWithMiddleware() in tests/Transpot/GuzzleTransportTest
• Added a configuration file for phpunit so that the test suite can be run
• Renamed to configuration files to *.dist (see here for more)
• Introduced CI pipeline to run psalm and unit tests

No breaking changes. Fixes the underlying issue addressed in #1 allowing OffersListDto::getNotes to not break if the notes property is null due to a strict string return type.

[ryancco]

/cc @OlegBabakov would love any feedback to get this to pass. Thanks!