feat: Add envconfig annotation for non-nested structs + env_prefix attribute

[Greesb]

Hello, I am new to Rust and thought of this feature while using it for another training project.
This should allow to parse environment variables that have a common prefix without having to use that prefix in every variables.

I also took the liberty to update the dependencies to their latest version, except for syn because it would require more work, because i feel like it's always better to have the latest updates for any dependencies if it might improve performance or patch some security issues.

I'll gladly take any input if the code, or even the logic, can be improved in any way.

[Greesb]

Added a commit to upgrade the syn dependency to its latest version

[greyblake]

@Greesb Thanks for your PR.

I left some comments.
Could you please also explain why you would need this feature or show a usecase where it solves a pain in your project?

[Greesb]

@Greesb Thanks for your PR.

I left some comments.
Could you please also explain why you would need this feature or show a usecase where it solves a pain in your project?

Thanks for the review, i'll handle them when i get back home.

The use case is that all of our application environment variables have a prefix, so we can avoid env variable "collision", and we want to use, in our application, those environment variables based on their names without the prefix since it's redundant, irrelevant and is way too much verbose.