add API version string to every response through a header

for easier debugging when looking at responses (especially when we have canary enabled), we should return the API's version string in every response through a header like `Server` (currently returns `cloudflare`) or a custom one like `X-Stacks-API-Version`:
```
Server: stacks-blockchain-api v3.0.3 (master:cd0c8aef)
```