Check if `block.url` is a valid Buf Build URL

[DenisCarriere]

Test on every PR

• [ ] Check if block.url is a valid Buf Build URL (Ex: buf.build/streamingfast/firehose-cosmos)
  • [ ] startsWith buf.build

Tests on release

• [ ] fetch URL contents
  • [ ] match if type exists in content

[YaroShkvorets]

Implemented in https://github.com/pinax-network/supported-networks/pull/21