<!-- Provide a brief summary of your changes -->
Add support for MCP extension negotiation during the initialize
handshake, as defined in the MCP extensions specification. Clients and
servers can now advertise supported extensions via a new `extensions:
Map<String, JsonObject>?` field on their respective capabilities
objects.
## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->
Extensions are supported part of the [MCP spec
](https://modelcontextprotocol.io/extensions/overview) and are available
in other language libraries e.g. [Golang
](862d78a265/mcp/protocol.go (L215)).
## How Has This Been Tested?
Existing and added unit tests
## Breaking Changes
<!-- Will users need to update their code or configurations? -->
Adding a new field to existing ClientCapabilities and ServerCapabilities
fields will break already compiled clients. They will need to recompile
on upgrading.
## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update
## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [x] I have added appropriate error handling
- [x] I have added or updated documentation as needed
## Additional context
<!-- Add any other context, implementation notes, or design decisions
-->
Co-authored-by: Pavel Gorgulov <devcrocod@gmail.com>
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