Allow cloning the transport with different credentials

[swallez]

Add Transport::clone_with_auth() that clones a transport (http client and connection pool) with a different authentication. The primary use case is for multi-tenant applications where multiple different credentials can be used to connect to the same Elasticsearch cluster.

To make it easier to set any kind of authentication header, this PR also adds a new kind of credentials, Credentials::AuthorizationHeader that accepts any string value.

[Copilot]

Pull Request Overview

This pull request adds a new method to clone the transport instance with different authentication credentials, enabling multi-tenant applications to efficiently switch credentials when connecting to the Elasticsearch cluster.

• Introduces the clone_with_auth() method to create a copy of the transport with a new set of credentials.
• Provides a test (clone_with_credentials) to verify that the new transport properly holds the new authentication data.
• Updates the transport implementation to support cloning of relevant fields, including the HTTP client and connection pool.

Comments suppressed due to low confidence (1)

elasticsearch/src/http/transport.rs:1317

• Consider renaming the test function 'clone_with_credentials' to 'clone_with_auth' for consistency with the API method name.

    fn clone_with_credentials() -> anyhow::Result<()> {