Feat:Implemented Stellar Asset Availability Monitor

[Endowed992]

Implemented Stellar asset availability tracking in:

stellar-asset-availability-monitor.ts
index.ts
stellar-asset-availability-monitor.spec.ts

What it does

monitors supported Stellar assets from provider discovery metadata
detects when assets are removed from all active providers
emits:
    status-change
    removed
    alert
    available

Validation

Ran Jest for the new monitor test file
Result: 3 passed

Closes #536

[mijinummi]

Hello @Endowed992 , thanks for your contributions, please kindly fix the build failure

[mijinummi]

Try run npm build then make a push . Well done @Endowed992