fix: various fixes around config (ChainSafe#433)

`config.zig` was broken in various places:

- `getValueUint64()` is a non-existent API, this shouldn't even have
  been merged (i was the reviewer so it's my bad)
- we were unnecessarily using an allocator when we could've just had
  fixed sized buffers for `config_name` and `blob_schedule`, both of
  which probably won't change that frequently anyway

Author: spiral-ladder