add banner property to member

[Lexedia]

Description

This also fixes a bug where if you pass empty query parameters, the resulted URL would look like something like this https://example.org?

• Add guild member banners discord/discord-api-docs#6986

Connected issues & other potential problems

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Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist:

• Ran dart analyze or make analyze and fixed all issues
• Ran dart format --set-exit-if-changed -l 160 ./lib or make format and fixed all issues
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works