Dynamic provider config

[sixlive]

Description

Closes #92

This PR adds support for overriding / updating the provider configuration on the fly. This is great for multi-tenant situations where users may bring their own API keys.

Prism::text()
    ->usingProviderConfig(['api_key' => '1234']);

Prism::structured()
    ->usingProviderConfig(['api_key' => '1234']);

These values are merged with the original provider config values so partial overrides are possible.

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