Fix errors for requests to specific WFS

[mholthausen]

This PR fixes four issues in superset_wfs_dialect that caused errors in superset when loading and cross-filtering charts based on specific WFS.

• WfsDialect.dbapi() and WfsDialect.import_dbapi() now return the concrete wfs_dbapi object from base.py and avoid errors like module 'superset_wfs_dialect' has no attribute 'paramstyle'
• filtered POST GetFeature requests now explicit UTF-8 payload so that cross-filter values containing special characters (e.g. Einschränkungen, Fußgängerverkehr) are transmitted correctly and no longer produce empty results. This fixes [Bug]: Crossfilter UTF-8 encoding/decoding issue #52 (46a0ca1).
• We keep [Bug]: COUNT funktioniert nicht auf bestimmtem WFS #53 open for follow-up discussion/decision on service fix vs. client-side workaround. This issue is not closed by this PR. The root cause is invalid JSON from WFS (trailing comma in geometry).
• if a WFS server ignores JSON output on POST and returns XML/GML, the dialect automatically retries using GET. This prevents JSON parsing failures such as unexpected character: line 1 column 1 caused by attempting to parse XML as JSON. This is a fallback. Requests may fail for long or complex filters (e.g. due to URL length). The root cause is a misconfiguration of the requested WFS service, which returns XML/GML instead of JSON for POST GetFeature despite requesting JSON (needs to be discussed). We keep [Bug]: Crossfilter funktioniert nicht auf bestimmtem WFS #54 open as this feature needs to be discussed further (46d5a85)

In addition, the README file will be updated to include instructions for integrating the dialect module into a superset development environment (372ded3).