Add functionality to get a File's content

[edif2008]

Summary

Starting with Connect 1.3.0, the users can get files that are stored in an Item. Connect Node SDK should enable the users to do that.

In this issue, we will get a file's content.

Tasks to be done

• [ ] Implement getFileContent function in the Items class in src/lib/resources.ts. The API endpoint that this function needs to call is v1/vaults/vaultId/items/itemId/files/fileId/content. You'll have to figure out the object type the promise expects.

  /**
   * Get an Item File's content.
   *
   * @param {string} vaultId
   * @param {string} itemQuery
   * @param {string} fileId
   * @returns {Promise<>}
   * @private
   */
  private async getFileContent(
           vaultId: string,
           itemQuery: string,
           fileId: string,
       ): Promise<> {
       // functionality
  }
  

  Note: Feel free to place this functionality in a separate class (e.g. Files) if you feel it's more suitable. This is just a suggested approach.
• [ ] Implement getFileContent function in OPConnect class in src/lib/op-connect.ts:

  /**
   * Get an Item File's content.
   *
   * @param {string} vaultId
   * @param {string} itemQuery
   * @param {string} fileId
   * @returns {Promise<>}
   */
  public async getFileContent(vaultId: string, itemQuery: string, fileId: string): Promise<> {
        return await this.items.getFileContent(vaultId, itemQuery, fileId);
  }
  

• [ ] Add a test for the new function in __test__/op-connect.test.ts:

  test("get file content", async () => {
      // actual test here
  }
  

[edif2008]

This issue is blocked by #62