Improve cmake

This MR improves the current CMake scripts in the following manner:

- Dependency management is now handled more consistently: system dependencies are found using `find_package()`, whereas source-only dependencies are found using `include_from_source()`. This new function integrates a dependency provided its source code is available. If it's not, it will try to download it from its git repository if it was defined.
- All external dependencies are now searched for in the top-level `CMakeLists.txt`.
- More consistent use of targets (we are closer to 100% modern CMake).
- Adds the `gkfs_feature_summary()` to allow printing a summary of all GekkoFS configuration options and their values.
- Moves all option definitions to `CMake/gfks-options.cmake`, so that the top-level `CMakeLists.txt` is cleaner. It also defines the `gkfs_define_option()` and `gkfs_define_variable()` functions to integrate the definition of options/variables with the automated reporting provided by `gkfs_feature_summary()`.

It also fixes #249.

Closes #211

Closes #211 and #249

See merge request !143

The function include_from_source() now checks if the source directory
for a external dependency is empty. If so, it will provide instructions
for the user to solve the issue (such as properly initializing
submodules).

Until #250 is fixed

This commit enforces replaces the use of absolute paths for several
targets in favor of using target_sources(), which allows specifying
source files for the same target in CMakeLists.txt files.

Also:
- Remove FindRocksDB.cmake, FindLZ4.cmake, FindJeMalloc.cmake.
Rationale: RocksDB already installs a RocksDBConfig.cmake that creates
all of these targets when calling find_package(RocksDB).

Remove obsolete submodules for CLI11.hpp and fmt.

Resolve "CI: Simplify coverage scripts"

This MR simplifies coverage generation in the following ways:

- Replaces the old `scripts/ci/coverage.sh` script with a new
 `scripts/dev/coverage.py` written in Python, making it more robust 
 and simpler to modify if needed.
- Adds specific CMake targets for generating coverage reports 
  directly from CMake.
- Adds specialized CMake presets to make it simpler to configure
  the different builds required to generate coverage information.
- Replaces `gcovr` with `lcov` + `lcov_cobertura`, since `gcovr` 
  exhibited some errors that were difficult to track down and 
  `lcov` worked out of the box.
- Adds HTML documentation about the coverage generation in 
  `docs/sphinx/devs/coverage.md`

This MR also updates the `v0.9.2` Docker images to update CMake
and include coverage-related packages. The `coverage` image in 
particular is no longer needed and has been removed.

Closes #252 #256 #257 #258

Closes #258, #257, #256, and #252

See merge request !163

- Rename job `coverage:zerocount` to `coverage:baseline`
- Move `coverage:baseline` job to `report` stage

GKFS_BUILD_TESTS and GKFS_INSTALL_TESTS are now set to `false` by
default.

Add 'attr' python module required by Sphinx

Image `testing:0.9.2` is now used for coverage generation.

- Upgrade CMake to 3.25.2 for consistency with other images
- Remove gcovr and install lcov
- Install loguru
- Install lcov_cobertura
- Install libgd-perl

Fixes #258

Fixes #257

Fixes #256

There are now explicit targets for generating coverage reports from
CMake itself:

- `coverage-zerocount`: Capture initial zero coverage data and write it to
  `${COVERAGE_ZEROCOUNT_TRACEFILE}`.
- `coverage-capture`: Capture coverage data from existing .gcda files and
  write it to `${COVERAGE_CAPTURE_TRACEFILE}`.
- `coverage-unified`: Merge any coverage data files found in
  `COVERAGE_OUTPUT_DIR` and generate a unified coverage trace.
- `coverage-summary`: Print a summary of the coverage data found in
  `${COVERAGE_UNIFIED_TRACEFILE}`.
- `coverage-html_report`: Write a HTML report from the coverage data
  found in `${COVERAGE_UNIFIED_TRACEFILE}`.
- `coverage-cobertura`: Write a Cobertura report from the coverage data
  found in `${COVERAGE_UNIFIED_TRACEFILE}`.

The new script is written in Python and is therefore more robust and
easier to modify if needed.

- Update cmake to v3.25.2
- Add ninja-build