[Feature] Set provider fallback based on credentials

[montezdesousa]

1. Why?

   • First time users are immediately blocked if they don't setup credentials before using openbb
   • v3 used to have a sources priority configuration

2. What?

Create a provider fallback mechanism that checks if required credentials are set before using a given provider.

The priority is the following:

I. Provider specified in the command (if you have no credentials it will display the same message as before)

obb.equity.price.quote("AAPL", provider="fmp") -> "fmp"

II. Configured priority list (or just 1 provider as before)

{
    "credentials": {},
    "preferences": {},
    "defaults": {
        "commands": {
            # 1 provider
            "equity.price.historical": { "provider": "fmp" },
            # priority list
            "equity.price.quote": { "provider": ["foo", "bar", "fmp"] }
        }
    }
}

obb.equity.price.historical("AAPL") -> "fmp"
obb.equity.price.quote("AAPL") -> "fmp" (tries "foo" and "bar" first)

III. Default priority is the list of providers for the command, sorted alphabetically

obb.equity.price.quote("AAPL") -> "yfinance" (tries "fmp" and "intrinio" first or other providers if they are installed)

IV. Raises exception

{
    "credentials": {},
    "preferences": {},
    "defaults": {
        "commands": {
            "equity.price.quote": { "provider": ["foo", "bar", "baz"] }
        }
    }
}

obb.equity.price.quote("AAPL")

OpenBBError: 
[Error] -> Provider fallback failed, please specify the provider or update credentials.
[Providers]
  * 'foo' -> not found
  * 'bar' -> not found
  * 'baz' -> not found

• Replaced routes by commands in the Defaults configuration class, since this feature is relevant to Python users only. Routes is an API concept. Will keep it as alias, accept slash split commands and set deprecation warning.

3. Impact:

   • Reduces friction for first time users. If the command supports a provider without credential it will be used, no setup needed.
   • LLMs can rely on configuration or just fallback to whatever provider works.

4. Testing Done:

   • Examples above and some combinations