feat: support StreamableHTTPClientTransport

[leo237]

Description

Adds support for Streamable HTTP. Related to #59

Fixes # (issue)

Type of change

• 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 not work as expected)
• Documentation update
• Refactoring (no functional changes)

How Has This Been Tested?

1. Tests are added
2. Manually tested with a Streamable HTTP server

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[benjamincburns]

Per the MCP spec, the streamable HTTP transport is meant to replace the SSE transport. Per their backward compatibility guide, clients should attempt to connect to URLs as though they are streamable HTTP first, and then fall back to SSE.

Doing this would fix another problem. Prior to this change the transport type can be inferred from the structure of the config entry. That is, the transport key of the config was optional, and should remain so. Supporting this fallback behavior would allow for that to continue.

[benjamincburns]

I pushed some updates for the above feedback, but I only had time last night to rework some of the tests. IIRC there are 3 tests still failing. We should also make updates to the README & examples to drop the transport field (it's optional and not necessary in 99% of cases).