feat: add native-tls as an optional TLS backend

[r0x0d]

Add reqwest-native-tls feature flag to allow users to choose between rustls (default) and native-tls for HTTP transports.

native-tls uses platform-native TLS implementations:

• OpenSSL on Linux
• Secure Transport on macOS
• SChannel on Windows

Motivation and Context

This is particularly useful for Linux distribution packagers who need to link against system TLS libraries (e.g., OpenSSL) rather than bundling a separate TLS implementation. Linking against system libs ensures security updates are applied system-wide and satisfies distribution packaging policies.

How Has This Been Tested?

Ran cargo test locally, but it seems that even the default upstream tests are failing right now.

Breaking Changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[Copilot]

Pull request overview

Adds an opt-in reqwest-native-tls feature to let consumers choose platform-native TLS (vs rustls) for reqwest-based HTTP usage, primarily to support distro packaging requirements.

Changes:

• Adds a new Cargo feature (reqwest-native-tls) that enables reqwest’s native-tls backend.
• Updates the crate README feature-flag documentation to describe available TLS backend options.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
crates/rmcp/Cargo.toml	Introduces reqwest-native-tls feature flag wiring to reqwest?/native-tls.
crates/rmcp/README.md	Documents TLS backend feature options for HTTP transports.

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[Copilot]

This section lists the TLS backend flags, but it doesn’t clarify (a) that they’re intended to be mutually exclusive and (b) that transport-streamable-http-client-reqwest / auth only enable the reqwest dependency (__reqwest) and still require exactly one of these TLS backend features to support https. Adding a short note about the required feature combinations would prevent misconfiguration and surprising runtime errors.

Suggested change

	These TLS backend features are intended to be mutually exclusive: enable at most one of them at a time. When using
	`transport-streamable-http-client-reqwest` or `auth`, those features only enable the `reqwest` dependency (`__reqwest`)
	and do not select a TLS backend for you; to support HTTPS, you must also enable exactly one of the TLS backend features
	listed above.

[Copilot]

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