Support for multiple configuration files in a directory

[osma]

Currently Annif only supports only a single configuration file, usually named projects.cfg (or projects.toml after #560 added support for the TOML format). This is sometimes a limitation, because there can be many independent projects (or groups of projects), but they all have to be in the same file. For example, if projects for different vocabularies are developed independently (perhaps in separate DVC repositories), it would ease their deployment if their configurations could be kept in separate files.

The proposal is to add support for a configuration directory. All the configuration files in that directory (filenames matching the patterns *.cfg or *.toml) would be read and their contents merged. This is similar to e.g. how the sites-available directory works in typical Apache HTTPD installations.

Annif already looks for files in the current working directory called projects.cfg and projects.toml, unless the path to the configuration file has been set using the -p CLI parameter or the ANNIF_PROJECTS environment variable. The search could be extended to look for a directory with a specific name. Of course, it should also be possible to use -p or ANNIF_PROJECTS to point at a specific directory.

The default name of the directory could be e.g. config, conf, conf.d, projects.d or something along those lines. (opinions?)