Support parameter override

**Describe the enhancement**
The config setup forces users to parse toml files or manually modify them to change things like rpc credentials.
Some users might not need modification of these, but in some setups these change continuously, forcing the user to use other tools to modify these files, adding additional dependencies to the setup.

**Use case**
I'm using bdk-cli mainly for testing. Maybe there is a good reason to not give too much freedom to the user to change parameters of their configuration, but I think at least RPC parameters are certainly prone to change, even more if the node is using cookie authentication.

**Additional context**
I updated the bdk-cli in the `bdk-sp` nix environment, and it broke most of the commands I was using to wrap bdk-cli. Some of the changes are useful, like writing keys into file, but some others are not, like rpc credentials. I'm starting and stopping this node very frequently and the rpc cookie changes each time. I had to add a tool like `dasel` to modify the parameters I need in the file.

Start with bdk-cli's config setup and the TOML/RPC parameter handling described in the issue, focusing on how RPC credentials and cookie authentication are loaded. Trace the relevant CLI entry points and verify that repeated node restarts can use changed credentials without manual file editing, while preserving existing configuration behavior.

索引モデルが issue の本文から書いたものです。