Add Apache 2.0 License

[ericapywang]

Problem

The repo currently does not have an Apache 2.0 license.

Solution

I added the license in the LICENSE file. I also updated the Cargo.toml file to specify the license type.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Infrastructure change (CI configs, etc)
• Non-code change (docs, etc)
• None of the above: (explain here)

Test Plan

No code was changed so tests should still pass.