Extend KBS to provide the resources required to create an encrypted overlay network

[cclaudio]

This PR requires https://github.com/confidential-containers/guest-components/pull/634 to be merged first.

This PR resolves #396 by adding the following changes:

• Add plugin interface
• Add nebula-plugin

Steps to test it

• enable nebula in the kbs-config.toml: enabled_plugins = ["nebula"]
• Generate User authentication key pair as in quickstart guide
• docker compose build
• docker compose up
• build kbs client: cd kbs && ATTESTER=snp-attester make cli && sudo make install-cli
• kbs-client --url http://127.0.0.1:8080 config --auth-private-key kbs/config/private.key set-resource-policy --policy-file kbs/sample_policies/allow_all.rego
• kbs-client --url http://127.0.0.1:8080 get-resource --path 'plugin/nebula/credential?ip[ip]=10.11.12.13&ip[netbits]=21&name=pod1' | base64 -d

The last command should return the credential requested, e.g.:

{
"node_crt":[45,45,45,45,45,66,69,71,73,78,32,78,69,66,85,76,65,32,67,69,82,84,73,70,73,67,65,84,69,45,45,45,45,45,10,67,109,69,75,66,72,66,118,90,68,69,83,67,89,50,89,114,70,67,65,56,80,47,47,68,121,106,83,117,113,83,49,66,106,67,97,49,113,88,69,66,106,111,103,83,85,66,117,100,74,104,101,84,75,105,49,122,50,86,66,77,102,112,66,10,68,103,105,120,77,56,87,81,90,55,97,81,109,99,114,68,114,53,52,53,47,50,74,75,73,68,116,87,73,57,109,43,70,98,120,80,119,107,52,54,122,71,90,88,116,56,70,103,112,74,52,71,52,120,88,57,76,97,108,56,78,55,98,116,10,120,65,43,68,69,107,65,76,110,114,80,88,87,88,116,51,113,102,77,109,87,48,102,110,68,88,97,111,48,90,104,112,108,54,65,104,49,115,82,47,115,115,120,120,80,56,109,99,108,99,78,87,101,110,47,76,82,84,68,48,112,101,122,68,10,105,101,76,82,75,79,70,109,121,104,111,74,107,118,66,73,86,109,53,82,104,54,43,56,68,89,115,68,10,45,45,45,45,45,69,78,68,32,78,69,66,85,76,65,32,67,69,82,84,73,70,73,67,65,84,69,45,45,45,45,45,10],
"node_key":[45,45,45,45,45,66,69,71,73,78,32,78,69,66,85,76,65,32,88,50,53,53,49,57,32,80,82,73,86,65,84,69,32,75,69,89,45,45,45,45,45,10,79,69,103,121,70,88,106,101,107,97,78,115,50,98,111,122,56,80,101,68,69,112,51,113,82,51,47,114,89,120,67,82,79,79,57,89,70,49,103,109,118,102,69,61,10,45,45,45,45,45,69,78,68,32,78,69,66,85,76,65,32,88,50,53,53,49,57,32,80,82,73,86,65,84,69,32,75,69,89,45,45,45,45,45,10],
"ca_crt":[45,45,45,45,45,66,69,71,73,78,32,78,69,66,85,76,65,32,67,69,82,84,73,70,73,67,65,84,69,45,45,45,45,45,10,67,107,115,75,71,85,53,108,89,110,86,115,89,83,66,68,81,83,66,109,98,51,73,103,86,72,74,49,99,51,82,108,90,83,66,76,81,108,77,111,109,43,43,103,116,81,89,119,109,57,97,108,120,65,89,54,73,76,54,55,105,73,115,79,10,51,83,68,71,76,67,49,84,121,54,78,82,77,121,48,57,56,56,84,110,53,51,77,47,71,88,112,80,77,84,99,57,66,43,85,73,81,65,69,83,81,80,75,67,107,97,99,72,79,89,86,121,66,104,57,69,102,106,74,107,65,110,48,72,10,81,73,72,102,114,106,51,83,76,48,67,118,49,77,81,109,81,98,107,55,99,89,116,81,70,82,114,111,104,57,51,104,121,55,99,81,55,112,51,99,56,105,86,110,67,56,109,80,97,107,113,70,47,101,66,84,48,82,111,67,69,119,111,61,10,45,45,45,45,45,69,78,68,32,78,69,66,85,76,65,32,67,69,82,84,73,70,73,67,65,84,69,45,45,45,45,45,10]
}