Blast API shutting down after Oct 31

Hey,

[Blast API](https://blastapi.io/) is shutting down after Oct 31, 2025. In [your code here](https://github.com/aleph-im/aleph-sdk-python/blob/main/src/aleph/sdk/conf.py#L108), you could replace it with a public endpoint from [ChainList](https://chainlist.org), or use [Chainstack](https://chainstack.com) for more reliable setups.