Add API Key Authentication

[dutchie032]

Added API Key authentication.

Due to a recurring question about authentication of clients I've implemented a Interceptor layer to the tonic server to check all calls for valid api keys.

example config:

auth.enabled = false --defaults to false
auth.tokens = {
    { client = "test", token = "Sometest" },
    { client = "another client", token = "Some other test" }
}

auth.tokens is a table of auth keys with their client name.
There can be a "default" client or a single key for all clients, but this is up to configuration.
There can be as many client keys as needed.

In the debug log the client that authenticates is logged.

Performance

Performance wise there is no notable difference.

Possible future features

Possibly in the future "expiration_date" can be added to automatically revoke issues, but I didn't think that was needed for a first implementation.

[rurounijones]

Can you update the README with a section specifically on Authentication with information on what clients should do (In general terms) to add authentication to their requests. (e.g. add the X-API-Key field to the header etc.)

[dutchie032]

Updated PR with example c# code.
It's collapsible so won't take too much space, but a dev can have a look at the code.
(I know there's Go clients and maybe even others, but I didn't want to add code that I didn't know to work)
Would be nice to have it added by someone if they implement it in Go or other languages.