text: Title: Add `robot.result.TestSuite.to/from_xml` methods
  Body: The current API we have for working with output.xml files programmatically requires using the `ExecutionResult` factory method. It returns a `Result` object that contains a `TestSuite` object in its `suite` attribute. The suite can be inspected and also modified, and the `Result` object has a `save` method for saving results back to XML after possible modifications. This is ok for many usages, but having a more convenient API for working with `TestSuite` objects would be nice.
  
  The result model gets `to/from_json` methods in RF 7.0 (#4847) and we could add `to/from_xml` to `TestSuite` as well. Similarly as the JSON counterparts, `to_xml` should return the XML by default, but also support writing it to a file. `from_xml` should work both with files and XML strings as well. We have all needed functionality for reading and writing XML available, so implementing these methods is easy. There are some design decision to be made, though:
  
  1. Should the `<suite>` structure returned by `to_xml` be wrapped with `<robot></robot>` the same way as in output.xml? I believe it's not a good idea at least by default. The suite anyway wouldn't have anything to write to `<errors>` and although `<statistics>` could be created, it would feel somewhat strange. Returning just the `<suite>` is also consistent with what `to_json` returns. Being able to create a full output.xml with something like `full=True` could be convenient, but it can be added later if needed.
  2. Should XML returned by `to_xml` contain the XML prolog (i.e. `<?xml version="1.0" encoding="UTF-8"?>`)? I believe it's not a good idea at least as the default behavior. It's optional in XML and could get in the way. Probably should be included if we supported something like `full=True`.
  3. Should `from_xml` support both "full" output.xml files and files containing only `<suite>`? I belive both should be supportred. Not supporting normal output.xml files would be *very* inconvenient and not supporting files generated by `to_xml` would be unacceptable. This is probably easiest to implement so that we in general support output.xml with and without `<robot>`.
  4. What to do if `from_xml` is used with an output.xml containing `<errors>`? Options include ignoring them, reporting them or raising an exception. Because `<errors>` doesn't really affect the final execution status, I believe we can ignore them as long as it's mentioned in the documentation. Adding something like ` ignore_errors=True` that can be set to `False` could be considered as well, but probably those who are interested in these errors can just use `ExecutionResult`.