compliance_tool: Avoid shipping schema files directly

[hpoeche]

The compliance_tool needs the metamodel schema files for JSON and XML, which are published in the admin-shell-io/aas-specs-metamodel under the CC-BY-4.0 license. It is not conform for us to ship these files (in compliance_tool/aas_compliance_tool/schemas) under MIT license.

I therefore suggest to remove these files from our repository and introduce two mutually exclusive options to the cli interface:

• --schema-path: This option can be used by users to provide a path to a directory that contains the schema files. The user is required to download the schemas himself. This allows for a offline usage of the aas_compliance_tool.
• --download-schema: If this option is chosen the compliance_tool downloads the schema files automatically from the source respoitory (similar to the CI runs) and caches them in a local directory for later runs. This provides a better user experience, if the tool is used to quickly check files.

We can discuss if any of the two options should be a default or if we require the user to actively choose.

[hpoeche]

A closer investigation on the usage of the schema files in the compliance tool showed that the files are only used in the
compliance_check_json.py and compliance_check_xml.py in two methods each called _check_schema().
These methods are never called by the compliance_tool but only in test cases.

In fact the option for using the compliance tool to check a file against the schema was removed with commit af73a4b in 2020.

[s-heppner]

Done in #549