Introduce embedded Dendrite server

[eyedeekay]

Introduce embedded Dendrite server, reduces custom code required for overlay networks

• Added a new embedded package to allow embedding a Matrix homeserver within Go applications.
• Created ServerConfig struct for configuration management of the embedded server.
• Created NewServer function to initialize the embedded server with provided configurations.
• Added Start and Stop methods for managing the server lifecycle.
• Updated main.go in dendrite-demo-tor and dendrite-demo-i2p to utilize the new embedded server.
• Added example usage in contrib/dendrite-demo-embedded/example/main.go.
• Updated README documentation for the embedded package with usage instructions and configuration details.

I previously submitted the original dendrite-demo-tor and dendrite-demo-i2p as eyedeekay, I'm not sure if the paperwork has changed at all but I assume I can submit this again under the same basic circumstances.

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