Update example to work with value overhaul

[tsachiherman]

Epic

stellar/stellar-cli#471

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What?

TL;DR: Update example to work with the ScVal overhaul.

This involves work across most of the dApp, but each item should be a fairly straightforward adaptation. A lot of the ScVal stuff is abstracted away, so fixing that abstraction is the bulk of the work:

• Update the underlying js-soroban-client library and use it correctly (related/blocking: update SCVal builders / helpers js-soroban-client#60)
• Update the convert.ts helpers to conform to the new format

[Shaptic]

@tsachiherman this is normally handled by the Core team (see the commit history); did you mean for this to apply to https://github.com/stellar/soroban-example-dapp/?

[tsachiherman]

@tsachiherman this is normally handled by the Core team (see the commit history); did you mean for this to apply to https://github.com/stellar/soroban-example-dapp/?

you're correct @Shaptic; I meant it to be soroban-example-dapp. I'll update that.