BlockFrostBackend does not work with prepod and preview testnets

[jb824]

Unable to setup backend using BlockFrost with project id for testnet (prepod). Sample.env file should be updated with .env variable used in backend.init.py, line 94. Resolving .env variable name causes a 403 status error. Testnet networks, prepod and preview, need to be included in a match..case or if..else in blockfrost.init.py for setting the network type in the base url.

        self.chain_context = BlockFrostChainContext(
            project_id,
            base_url=ApiUrls.mainnet.value, # causes a 403 status code error with testnet project_id
        )
        self.api = self.chain_context.api
        self._block_cache: dict = {}

A match...case like below would help resolve this issue

        url = ""
        match project_id[:7]:
            case "prepod":
                url = ApiUrls.prepod.value
            case "preview":
                url = ApiUrls.preview.value
           ...     

I can submit a pull request tomorrow or next week unless this was left intentionally.