Add a "Sign in with Web3" option

[nya-elimu]

Implement Web3 sign-on functionality similar to what has already been implemented for the webapp: https://eng.elimu.ai/sign-on

Corresponding REST API endpoint for signing on with Google in the Android app: https://github.com/elimu-ai/webapp/blob/main/src/main/java/ai/elimu/rest/v2/crowdsource/ContributorsRestController.java

Corresponding controller in the webapp for signing on with Web3 via a browser: https://github.com/elimu-ai/webapp/blob/main/src/main/java/ai/elimu/web/SignOnControllerWeb3.java

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Depends on elimu-ai/webapp#1544

[nya-elimu]

@hhio618 Note that the ContributorsRestController currently only supports signing in with Google. So it expects providerIdGoogle to be included in the request body.

Therefore, we should create a separate REST controller class for signing in with Web3. I'll handle that in a separate GitHub issue: elimu-ai/webapp#1544

[nya-elimu]

@hhio618 In the Delta app, you'll see that they launch the MetaMask app (in a separate Android Activity)
for fetching an Ethereum address:

Do you think we could do something similar?

[hhio618]

@nya-elimuai Sure, we can do this too!