Add new API integration: /dowjones-constituent

[sorndotdev]

Instructions
Replace all example values below with your actual implementation details. Do not keep the examples as-is. Make
sure to read the full documentation before contributing.

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FMP Docs Link:
Replace with the direct link to the relevant endpoint in the Financial Modeling Prep docs

https://site.financialmodelingprep.com/developer/docs/stable/dow-jones

Parameters:
Replace with the required and optional endpoint parameters (or N/A)

• Required: N/A
• Optional: N/A

Proposed model class name:
Replace with the name of the model class you plan to add

FmpDowJonesConstituent

Proposed client class name:
Replace with the name of the client class

FmpEtfClient

Proposed client method names:
Replace with the method names in the client

• constituents().dowJones()

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Requirements

All models

• start with Fmp
• are implemented as Java 16+ records
• implement the FmpModel interface
• have serialVersionUID
• have test: is_serializable
• have test: serializes
• use date time classes for dates, instead of strings, where appropriate
• use value objects, instead of strings, where appropriate
• have *TestData in testFixtures

All services

• use the model name + Service suffix
• are added to the appropriate FmpClient
• have tests with real responses from FMP
• with responses stored as JSON in testFixtures (for large responses, include only an excerpt)

General

• aim for 100% test coverage; TDD is preferred
• double-check your work before creating a PR
• refactor unrelated code in a separate PR from feature or bugfix changes (don’t refactor "as you go")
• adhere to the existing code style

[sorndotdev]

FMP JSON response excerpt added in 63891199.