Allow users to override token export environment variable names

[beubax]

What happened?

Authsome currently exports tokens using default environment variable names. Defaults are useful, but some tools, SDKs, scripts, or agent runtimes expect credentials under a different env var name.

When the expected env var name differs from Authsome's default, users have to add manual shell glue or wrapper logic outside Authsome.

What did you expect?

Users should be able to configure the environment variable name used when exporting a token. Authsome should continue to provide sensible defaults, but users must be able to override the export name when a provider, connection, or consuming tool requires a different variable.

The override should be reflected consistently wherever tokens are exported or injected into command environments.

Steps to reproduce

1. Configure or log in to a provider with Authsome.
2. Run an export flow or a command that depends on exported credentials.
3. Try to use a tool that expects the token under a non-default env var name.
4. Observe that Authsome does not provide a clear way to override the exported env var name.
5. Add configuration support so users can keep the default or specify a custom env var name for token export.

[manojbajaj95]

why is this needed?

[beubax]

Was trying to use authsome with printing press library and each cli expects a token in the env with a slightly different name than what we have already in authsome's provider definitions. There's no easy way to modify these apart from creating entirely new providers (not even possible on hosted mode).