feat: introduce declarative function signatures (GoogleCloudPlatform#116

)

chore: update CONTRIBUTING.md with releasing process (GoogleCloudPlat…

…form#109)

chore: allow guzzle/psr 2.0 (GoogleCloudPlatform#102)

fix: update background to CloudEvent conversion (GoogleCloudPlatform#73)

fix: allows json body with binary event (GoogleCloudPlatform#63)

feat: add better handling for exceptions (GoogleCloudPlatform#62)

docs: adds CloudEvent helloworld to examples, README, and tests (Goog…

…leCloudPlatform#55)

fix: better type-handing in Context class (GoogleCloudPlatform#47)

test: use github actions (GoogleCloudPlatform#41)

feat: uses guzzle/psr and psr-7 instead of http-foundation (GoogleClo…

…udPlatform#28)