soroban-rpc: Add support for configuration from a file

[paulbellamy]

What problem does your feature solve?

As an operator, I'd like to be able to configure soroban-rpc from a file, so that it is easier to deploy automatically via puppet/terraform/k8s and such.

What would you like to see?

Add support for reading config from a file. Follow the patterns we use for Horizon, so it is familiar.

What alternatives are there?

Details

Create a toml configuration that would include the local configuration options. The configuration file need to be versioned, and support configuration upgrades. The effective configuration in which the soroban-rpc would work with would be determined by ( highest precedence first ):

1. command line
2. configuration file
3. default configuration setting

The configuration file should internally contain a section which would be "transported" onto the captive core. That would allow the operator to configure the behaviour using a single file.