Plugin System for Third-party Parsers

[CodeMaverick-143]

Users may need to parse proprietary formats (e.g., HL7, ISO8583).

Technical Implementation:

• Define a Parser interface: (req) => Promise<any>.
• Add app.registerParser(contentType, parserFn).
• Update the content-type detection logic to check registered custom parsers first.

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Acceptance Criteria:

• A user can register a custom parser for application/x-custom.
• Requests with that content type have req.body populated by the custom parser.