gRPC server and client documentation

[fntlnz]

/area documentation

What would you like to be added:
We need to document:

• Falco gRPC server configuration and usage
• Falco gRPC outputs configuration and usage
• gRPC API documentation
• How to generate the certificates for mutual TLS
• How to use the SDK with examples

Why is this needed:

Because it's a new feature and people will want to use it!

[leodido]

In order to try out the feature you need to generate the certificates. Atm you can use the following script:

#!/bin/bash

# Generate valid CA
openssl genrsa -passout pass:1234 -des3 -out ca.key 4096
openssl req -passin pass:1234 -new -x509 -days 365 -key ca.key -out ca.crt -subj  "/C=SP/ST=Italy/L=Ornavasso/O=Test/OU=Test/CN=Root CA"

# Generate valid Server Key/Cert
openssl genrsa -passout pass:1234 -des3 -out server.key 4096
openssl req -passin pass:1234 -new -key server.key -out server.csr -subj  "/C=SP/ST=Italy/L=Ornavasso/O=Test/OU=Server/CN=localhost"
openssl x509 -req -passin pass:1234 -days 365 -in server.csr -CA ca.crt -CAkey ca.key -set_serial 01 -out server.crt

# Remove passphrase from the Server Key
openssl rsa -passin pass:1234 -in server.key -out server.key

# Generate valid Client Key/Cert
openssl genrsa -passout pass:1234 -des3 -out client.key 4096
openssl req -passin pass:1234 -new -key client.key -out client.csr -subj  "/C=SP/ST=Italy/L=Ornavasso/O=Test/OU=Client/CN=localhost"
openssl x509 -passin pass:1234 -req -days 365 -in client.csr -CA ca.crt -CAkey ca.key -set_serial 01 -out client.crt

# Remove passphrase from Client Key
openssl rsa -passin pass:1234 -in client.key -out client.key

I suggest to generate certificates in /tmp to match the go client example paths and then:

1. modify falco.yaml configuration file to point to /tmp/server.key etc.
2. use the falcosecurity/client-go example, eg., go run example/output/main.go