feat: generate and use private key from environment variable and add support for dnsaddr seed peers

[ksrichard]

Description

In order to generate a private key earlier that can be used at any start from env vars, we needed a solution.
Now there is a new cli option called --generate-identity. With this option, you can generate a new identity (an encoded libp2p private key) that can be used to set value later for SHA_P2POOL_IDENTITY environment variable (the command outputs a json where private_key field is the one need to use).
If that var is set, when starting p2pool with --stable-peer flag, it will look for first for this env var and tries to decode and use it.
Also /dnsaddr/* seed peer addresses were not supported, now it is supported, so p2pool will parse all the TXT DNS records and add them to kademlia to bootstrap.

Motivation and Context

How Has This Been Tested?

cargo build --release
export SHA_P2POOL_IDENTITY=$(./target/release/sha_p2pool --generate-identity | jq -r '.private_key')
./target/release/sha_p2pool --stable-peer

After starting/stopping many times p2pool, the peer ID remains the same.

What process can a PR reviewer use to test or verify this change?

See testing.

Breaking Changes

• None
• Requires data directory on base node to be deleted
• Requires hard fork
• Other - Please specify