fix: Add proper defaults for RpcServiceOptions

[FrederikBolding]

Explanation

Add proper defaults for fetch, btoa, and isOffline in RpcServiceOptions.

References

https://consensyssoftware.atlassian.net/browse/WPC-1055

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

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Note

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[FrederikBolding]

This was erroneously overriding any isOffline implementation passed in via getRpcServiceOptions. Instead, we now use it as a default.