feat: add optional fallbackNodeUrls property to RpcProvider

[piwonskp]

Motivation and Resolution

Solve issue #872. The PR replaces #905. PR targetting v6: #953

RPC version (if applicable)

...

Usage related changes

Development related changes

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 should be a discussion about the design for this feature. The main issue I see with this approach is that by being applied after the error handler the fallback would also take effect for expected error responses, e.g. a response with HTTP status 200 that contains a Transaction hash not found error, which doesn't feel right to me.

[tabaktoni]

@piwonskp Will you update for expected error responses?

[piwonskp]

@tabaktoni @penovicp Sure, I'll narrow down error handling to fetch errors

[piwonskp]

@tabaktoni @penovicp Done, rpc error handling is done outside of the catch clause now