API: ignore charset when checking content types

We still expect all communication to be in UTF-8, but we really don't
need a charset attribute for that.

fixes #4218, replaces #4219

fix error handling in XMLRPC

response errors in call() need to be returned as Error not as thrown
Exception.

API: remove file and date transformations

We always deal with timestamps or base64 now. Removing some of the magic
makes everything less complex.

Only affected plugin is confmanager, which needs an

API: remove file and date transformations

We always deal with timestamps or base64 now. Removing some of the magic
makes everything less complex.

Only affected plugin is confmanager, which needs an update.

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codestyle adjustments: function declaration braces/spaces

Apply rector fixes to inc/Remote

check XMLRPC mime type case-insensitive

enforce content type on XML-RPC. addresses #3991

This ensures only text/xml or application/xml content types are accepted
when communicating with the XML-RPC API

CORS on XMLRMPC API

This allows setting a CORS header to make the xmlrpc API accessible from
JavaScript clients directly in the browser.

abort XMLRPC later in serve()

Fix #3619 and correctly handle exceptions in XMLRPC

This ensures that any exception happening during XMLRPC processing is
signalled correctly to the client as XML encoded error message.

move IXR XML RPC to composer dependency #1970

This is a monky patched version of kissyfrot/php-ixr until my PR has
been accepted upstream.

Plugins using the XML RPC client may need adjustments.

Add failing unit test for #2883

moved XMLRPC server class from lib/exe to inc/Remote