Add I2P Support

**Is your feature request related to a problem? Please describe.**
I'm a bit frustrated because Zeronet only works over Tor. Tor is - depending on your point of view - somewhat centralized (by the servers knowing the nodes and bridges). So it would be nice to add another option as well.

**Describe alternatives you've considered**
Support for another dark web. The best known are Tor, I2P and Freenet. Only Tor and I2P also support data transport. Tor support is already included.

**Describe the solution you'd like**
My suggestion would be the Darknet I2P. This is designed more for internal services and has no exit nodes (without further configuration).

**Additional context**
I2P offers two interfaces - BOB and SAM. However, BOB is considered outdated by the official team (still supported by i2pd). Both offer the SAM interface, which is simple but very functional. Furthermore there are two I2P implementations Java I2P and i2pd.

Java I2P is the official implementation. It is being developed further by various developers.
i2pd is an implementation of I2P written in C++. It was originally written by Russian developers.
In the meantime, both teams are coordinating and talking to each other. The implementations are compatible with each other.

**Links**
- https://geti2p.net/
- https://i2pd.website/ / https://i2pd.xyz/
- https://geti2p.net/en/docs/api/samv3

The issue names no files or tests. Start by reviewing ZeroNet’s existing Tor support and the linked I2P SAM v3 documentation, then compare the integration requirements for Java I2P and i2pd. Done would require an agreed interface and working I2P transport support.

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