Add basic http client support

[jcat4]

Motivation and Context

Closes #3

Follow-up to #27

Happy to close the other PR and just keep this one if we want.

This adds the ability to build MCP clients with the ruby sdk. I've started with a basic HTTP client. We can add other things (i.e. streamable HTTP) later.

For simplicity, I'm just allowing custom headers to specify auth. I didn't want to build an abstraction around different auth types prematurely.

How Has This Been Tested?

The local gem build has been tested in 2 different internal repositories and is working as expected so far.

Breaking Changes

Just the stuff from #27

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

[jcat4]

Decided to build in auth, going to draft to test it

[jcat4]

Auth is tested and working

[topherbullock]

@jcat4 can you rebase on main and resolve conflicts? seems to be largely the model_context_protocol => mcp rename