Make xsdata optional and add requests-only fallback in FiscalClient

[rvalyi]

Motivation

• Reduce friction for consumers by making xsdata a soft (optional) dependency so basic users need only requests + requests-pkcs12.
• Allow FiscalClient to perform basic SOAP requests using plain XML when xsdata is not installed, increasing adoption for lightweight use-cases.
• Preserve existing behavior and compatibility for projects that do use xsdata (nfelib / Odoo integrations) by keeping parsing/databinding unchanged when available.

Description

• Remove xsdata from mandatory dependencies and expose it as an optional extra xsdata in pyproject.toml, and document the optional extra and a requests-only usage example in README.md.
• Add runtime detection (importlib.util.find_spec) and conditional imports so the module can operate with or without xsdata, providing lightweight fallback classes and exceptions when missing.
• Implement a requests-based fallback in FiscalClient that uses requests.Session when xsdata is unavailable, including helpers _session, _post, prepare_headers, and _webservice_name, and a _xsdata_available flag to route behavior.
• Adjust prepare_payload/send to accept raw XML payloads (str/bytes or {'raw_xml': ...}) in no-xsdata mode and to return the raw SOAP XML response when parsing is not available; update tests to cover the fallback behavior.

Testing

• Ran ruff check brazil_fiscal_client tests and the code style/lint checks passed.
• Compiled the package sources with python -m compileall brazil_fiscal_client tests and compilation succeeded.
• Ran pytest -q in this environment but test collection failed because the optional test dependency nfelib is not installed here (this is unrelated to the change and affects collection of xsdata-dependent tests).
• Added automated tests for the no-xsdata fallback in tests/test_fiscal_client.py, and a small automated smoke check invoking FiscalClient in requests-only mode completed successfully in this environment.

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Codex Task