Add GitHub Actions for CI

[nellshamrell]

Should run unit, functional (if applicable), and e2e tests

• Basic GitHub Actions workflow with cargo build (Add GitHub Actions workflow ci.yaml #34)
• Run unit tests (Add GitHub Actions workflow ci.yaml #34)
• Run cargo fmt for coding style (Add GitHub Actions workflow ci.yaml #34)
• Run cargo clippy for linting (Add GitHub Actions workflow ci.yaml #34)
• Run functional tests
• Run e2e tests

[dongsupark]

Depends on #30.

[dongsupark]

Split into 6 different tasks.
I would like to start working on the first 4 tasks, GitHub Actions with unit tests, cargo fmt & clippy, which should be low-hanging fruits.

[dongsupark]

The first 4 tasks are done via #34.