Replace sdk_config_customizer with specialized configuration APIs

[dkropachev]

The generic sdk_config_customizer hook exposes raw SDK config kwargs and makes feature-specific APIs harder to document and validate.

Proposal:

• Deprecate sdk_config_customizer.
• Replace known use cases with typed Alternator config fields.
• Keep SDK Config ownership inside the client so endpoint routing, auth, retries, TLS, and final wire headers remain controlled by Alternator.
• Add migration docs for existing customizer users.

Known replacements:

• User-Agent customization should use a dedicated callback field, not sdk_config_customizer.
• Retry, timeout, pool size, AWS region, and TLS client certs already have first-class config fields.

Follow-up to #61.