build: use `GTest` CMake module to handle dependencies on `gtest`

See merge request asapo/asapo!221

Fix doctstring for create_consumer/create_producer

See merge request asapo/asapo!210

As a consequence, we can remove code in CMake files to handle these
dependencies on `gtest` and `gmock`, which fixes the following error
when running CMake:

```sh
$ cmake -G Ninja ../.. -DPython_EXECUTABLE=python3.7 -DBUILD_TESTS=ON -DENABLE_COVERAGE=ON -DBUILD_CPP_DOCS=ON -DBUILD_PYTHON_DOCS=ON
...
CMake Error at CMakeModules/testing_cpp.cmake:47 (IF):
  if given arguments:

    "WIN32" "AND" "STREQUAL" "Debug"

  Unknown arguments specified
Call Stack (most recent call first):
  common/cpp/src/common/CMakeLists.txt:11 (add_plain_unit_test)
```

Merge branch 'fix-errors-when-compiling-from-a-directory-that-is-not-a-direct-subdirectory' into 'develop'

ci: fix errors when compiling from a directory that is not a direct subdirectory

See merge request asapo/asapo!219

ci: exclude Mongo DB downloaded files from Git repository

See merge request asapo/asapo!214

CMake was not generating the files `version.h` and `version_lib.go` into
the project source directory when building from a subdirectory such as
`./build/debug/`. So these files could not be found during compilation.

Rationale: `git` commands such as `git status` don't list these files
anymore which makes the output much more readable.

Move part related to broker request to common.

See merge request asapo/asapo!211

ci: use Python interpreter found by CMake when building sphinx doc

See merge request asapo/asapo!220

Rationale: using possibly different Python interpreters for the
different task can make the installation of packages required by the
different interpreters more complicated.

ci: run a single GitLab CI pipeline per commit

See merge request asapo/asapo!215

Commit to fix the current behavior of GitLab that triggers two CI
pipelines for the same commit.

For example, both the pipelines
[114510](https://gitlab.desy.de/asapo/asapo/-/pipelines/114510) and
[114511](https://gitlab.desy.de/asapo/asapo/-/pipelines/114511) ran on
the same commit 56d16797. See
asapo/asapo!214

Rename log field id to meta_id in case of get_metadata request. This field is string in contrast to message id, which is long.

Try to crate/get auto_id again in case of problem. This does happen when several receivers create new collection.