chore: use thegraph-headers graph-attestation http header

[LNSD]

This pull request includes several changes aimed at improving the handling of HTTP headers and simplifying the codebase. The most significant changes include the addition of a new crate for headers, removing the attestation_header module, and updating how headers are added to HTTP responses.

Improvements to header handling:

• Cargo.toml: Added the thegraph-headers crate with the attestation feature.
• src/client_query.rs: Updated imports to use thegraph_headers for GraphAttestation and HttpBuilderExt. Removed the attestation_header module import. [1] [2]

Codebase simplification:

• src/client_query.rs: Refactored the handle_query and handle_indexer_query functions to conditionally add the GraphAttestation header only if it is present. [1] [2]
• src/client_query/attestation_header.rs: Removed the attestation_header module, moving its functionality to the thegraph-headers crate.
• src/http_ext.rs: Removed the HttpBuilderExt trait as it is now provided by the thegraph-headers crate. (src/http_ext.rsL1-L25R1)
• src/main.rs: Removed the http_ext module import.

[Theodus]

LGTM