feat(api): support quotes to amount

[cruzdanilo]

this pr adds support for fetching quotes by specifying the desired toAmount instead of fromAmount.

this is achieved by:

• introducing a new union type for QuoteRequest that allows either fromAmount or toAmount (but not both).
• updating the getQuote method in src/services/api.ts to:
  • validate that either fromAmount or toAmount is provided, and not both.
  • construct the api url to hit the /quote/toAmount endpoint when toAmount is specified, or the existing /quote endpoint when fromAmount is used.
• updating the unit tests in src/services/api.unit.spec.ts to cover the new validation logic for toAmount and the case where both fromAmount and toAmount are erroneously provided.

this aligns the sdk with the v1/quote/toAmount api endpoint (docs: https://apidocs.li.fi/reference/get_v1-quote-toamount), providing users with more flexibility when requesting quotes.

this is my first contribution. any feedback is welcome.