24 phase 4 prototype add brl off ramp support

Closes #24

This PR introduces BRL token off-ramping, making the prototype general to the tokens defined in it's configuration.

Based on the changes introduced in the prototype script version.

Add a configuration file with the information for the available tokens, both in the UI and backend service (since in the future these may be in a separate repo)
Changes to fetch all the balances from the chain and check which one can the user withdraw based on minimum limits, disable tokens accordingly.
Adds button selection.
Fetch the token config in the signing service for correct signing based on a corresponding extra parameter sent as a request from the UI (example). Asset Id sent to the service is the asset code string used in Stellar, and the corresponding config will be matched by it.
Case handling of the memo.
Other changes required to propagate the selected token id configuration.

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pendulum-chain / vortex