Upgrade to Actix Web 4; support of env config interpolation

[aegenet]

Hello, this PR fixes an issue and adds a feature (I can split these into two PRs if you prefer)

• Upgrade of Vigil dependencies, particularly Actix Web from 3 to 4:

This will fix issue #117, which I encounter every day.

Resolves #117

• Support for environment variables with string interpolation in the configuration file.

For example: manager_token: ${VIGIL_MANAGER_TOKEN}.

[valeriansaliou]

Thank you so much! Will issue a release soon.

[williamdes]

Thank you so much ! This is a very important feature for private URLs in public repo config instead of having to do it manually: https://github.com/datacenters-network/status/blob/3005c9b0eb447df2ab8d1af1d2587618444086d2/docker-entrypoint.sh#L7