feat: add optional fallbackNodeUrls property to RpcProvider

[jereldlimjy]

Motivation and Resolution

Solve issue #872

Usage related changes

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Development related changes

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Checklist:

• Performed a self-review of the code
• Rebased to the last commit of the target branch (or merged it into my branch)
• Linked the issues which this PR resolves
• Documented the changes in code (API docs will be generated automatically)
• Updated the tests
• All tests are passing

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[penovicp]

There is a notable change with the channel/provider split in the upcoming v6 version that relates to how underlying requests are made, so adding this feature should be based on and the PR should target the beta branch.

[eliotstock]

@penovicp, we'd love to use this quite soon in some downstream dApps.

When do you see v6 being released?

Could we do a v5 version in parallel, do you think?

[ivpavici]

hello!

I want to clarify the steps for this PR!
First, @eliotstock you would like this PR to be included into Starknet.js v5? If that is the case, it's possible!
Second, for Sn.js v6, @jereldlimjy should modify it a bit and target the beta branch, do you want to do it?

Also, adding a test would be awesome!

[penovicp]

Superseded by #980