Add a l10n service to the web-based API (#1047)

## Problem

This PR aims to implement a `l10n` API in the web-based API. Its design
could be the base for other services.

Dropping the D-Bus interface is out of scope. 

## Tasks

- [x] Export the list of locales
- [x] Export the list of keymaps
- [x] Export the list of timezones
- [x] Allow setting the locale
- [x] Allow setting the keymap
- [x] Allow setting the timezone
- [x] Allow setting the UILocale
- [x] Read current configuration (locale, keymap, timezone)
- [x] Read the UILocale
- [ ] Write all changes or nothing (let's postpone when we drop the
D-Bus API as it will be easier).
- [x] Emit a signal when UILocale changes
- [x] Add unit tests
- [x] Add some integration test
- [x] Initialize the locales properly
- [x] More compact output (partially done; let's work on that when we
start working in the web UI).
- [x] Enable compression

## Screenshots

![emitting
events](https://github.com/openSUSE/agama/assets/15836/01f3c3cf-bcf9-4cf4-95d5-787d02b7a380)

## Links

Trello:
https://trello.com/c/sjR61cpk/3556-5-expose-the-l10n-api-over-http

.github/workflows/ci-rust.yml

@@ -76,6 +76,7 @@ jobs:
         pam-devel
         python-langtable-data
         timezone
+        xkeyboard-config
 
     - name: Install Rust toolchains
       run: rustup toolchain install stable
@@ -92,10 +93,12 @@ jobs:
         tool: cargo-binstall
 
     - name: Install Tarpaulin (for code coverage)
-      run: cargo-binstall --no-confirm cargo-tarpaulin
+      run: |
+        echo "$PWD/share/bin" >> $GITHUB_PATH
+        cargo-binstall --no-confirm cargo-tarpaulin
 
     - name: Run the tests
-      run: cargo tarpaulin --out xml
+      run: cargo tarpaulin --out xml -- --nocapture
 
     # send the code coverage for the Rust part to the coveralls.io
     - name: Coveralls GitHub Action

rust/agama-dbus-server/Cargo.toml

@@ -29,7 +29,7 @@ macaddr = "1.0"
 async-trait = "0.1.75"
 axum = { version = "0.7.4", features = ["ws"] }
 serde_json = "1.0.113"
-tower-http = { version = "0.5.1", features = ["trace"] }
+tower-http = { version = "0.5.1", features = ["compression-br", "trace"] }
 tracing-subscriber = "0.3.18"
 tracing-journald = "0.3.0"
 tracing = "0.1.40"
@@ -47,6 +47,7 @@ chrono = { version = "0.4.34", default-features = false, features = [
   "clock",
 ] }
 pam = "0.8.0"
+serde_with = "3.6.1"
 
 [[bin]]
 name = "agama-dbus-server"