fix: proper crate layout and resolve all clippy warnings

[shivv23]

Summary

Split the crate into proper library layout with lib.rs as the canonical module export surface, and resolve all clippy warnings to make the PR mergeable.

Changes

Crate Layout:

• Add lib.rs as library root with module exports (aws, azure, gcp, digiocean, errors, traits, types)
• Simplify main.rs to minimal binary entrypoint
• Export token_provider module in traits
• Make aws_dns and aws_loadbalancer modules public for tests
• Remove non-existent module declarations (aws_paas, aws_nosqlindexed, aws_rbmds)

Clippy Fixes:

• Remove unused imports (CreateClusterError, DeleteClusterError, LlmStreamEvent)
• Fix deprecated aws_config::from_env() → defaults()
• Fix unnecessary mut in azure_auth.rs
• Fix io::Error::new → io::Error::other
• Add #[allow] for API-compatible naming (snake_case, camelCase from GCP APIs)
• Add #[allow] for too_many_arguments on thin SDK wrapper functions
• Add #[allow] for new_without_default where constructors need config
• Add #[allow] for dead_code on response structs

Validation

• cargo clippy --lib --all-features -- -D warnings ✓ passes
• cargo check --lib ✓ passes
• cargo check ✓ passes

[Sandipmandal25]

hi @shivv23
This is the same pattern instead of fixing the existing PR a new one has been opened. PRs #73, #121, #123 and now #124 are all open and doing overlapping things. Please close the older ones and consolidate into a single PR, otherwise this creates and unmanageable number of conflicts for everyone working on the repo. please discuss things before opening again a new consolidated pr. Thank you so much for trying to resolve the issue.

[Sandipmandal25]

same pattern again this change does not belong to this pr.

[shivv23]

@Sandipmandal25 Thanks for your review and appreciation! Have done the required tasks. Thanks again!