feat!: implement complete password auth chain (+)

This is a final implementation, it completely works, is safe and
audited. There are several changes overall (most explained in the
README):
- Added `debuggable` cargo feature to disable debug ability, reducing a
  bit library size.
- Completed README with setup, concernsm, features and more.
- Add `asdeny` argument to behave as a PAM barrier. Updated nixos
  configuration accordingly.
- Modularity, separated config from API client and CURL client, each can
  take a `debug` option for a clean function call interface.
- Unified logging methods, there is a current `e/println!` call in all
  the codebase, behind a macro, to allow for consistent log formatting.
- Readability, separated and broke down cognitive complicated structures
  for ease of readability, methods are as simple as I could get them to
  be and reusable across `authentication` and `password` flows.
- HTTP headers now are macro'ed to prevent typos and have unified casing
  and representations.
- Simplified CURL interface so it integrates perfectly with rust type
  system for just what's necessary, it's more readable and loggable.
- Checked PAM returned errors to see if they are sensible enough.
- Checked this interacts with `pam_unix.so` as expected.

Cargo.toml

@@ -11,6 +11,10 @@ categories = ["authentication", "network-programming"]
 [lib]
 crate-type = ["cdylib"]
 
+[features]
+default = ["debuggable"]
+debuggable = []
+
 [profile.release]
 codegen-units = 1
 opt-level = 3