Add support for setting the filter language via CLI

[torse]

Related Issue(s): #139

Description:

This PR add support for setting the filter language with a new cli parameter --filter-lang. If not set, it defaults to cql-json. This allows for using e.g. cql-text filter by specifying

--filter 'eo:cloud_cover < 60' 
--filter-lang "cql2-text"

PR Checklist:

• Code is formatted
• Tests pass
• Changes are added to the CHANGELOG

[torse]

I intentionally kept the default value of the --filter-lang as cql-json to keep the backwards-compatibility. However according to the filter extension spec, the default value of the filter language depends on the method:

If undefined, defaults to cql2-text for a GET request and cql2-json for a POST request.

Would you like the defaults of the CLI parameters to be updated accordingly?