[Feature request] Allow re-compute headers on each grpc call

When running grpc requests using bearer authentication I need to provide the bearer token in the header in this manner:

```
grpcui -H "Authorization: Bearer $(cat /path/to/token)" ...
```

In my environment the tokens are short lived for security and are automatically regenerated at `/path/to/token`. Currently the token expires in 30 minutes and I need restart the grpcui web server every 30 minutes to continue to use it, otherwise requests fail authentication. It would be useful if grpcui could optionally allow some headers to be recomputed on each request, so that the same instance of the server can be used for a while.

No file, test, or entry point is named in the issue. Start by tracing how grpcui parses -H headers and applies them to each gRPC request. Done means an optional header can be recomputed for every request, newly generated bearer tokens are used without restarting the web server, and existing static headers still work.

Written by the indexing model from the issue text.