fix default network handling (#2029)

## Issue Addressed
#1992 and #1987, and also to be considered a continuation of #1751

## Proposed Changes
many changed files but most are renaming to align the code with the semantics of `--network` 
- remove the `--network` default value (in clap) and instead set it after checking the `network` and `testnet-dir` flags
- move `eth2_testnet_config` crate to `eth2_network_config`
- move `Eth2TestnetConfig` to `Eth2NetworkConfig`
- move `DEFAULT_HARDCODED_TESTNET` to `DEFAULT_HARDCODED_NETWORK`
- `beacon_node`s `get_eth2_testnet_config` loads the `DEFAULT_HARDCODED_NETWORK` if there is no network nor testnet provided
- `boot_node`s config loads the config same as the `beacon_node`, it was using the configuration only for preconfigured networks (That code is ~1year old so I asume it was not intended)
- removed a one year old comment stating we should try to emulate `https://github.com/eth2-clients/eth2-testnets/tree/master/nimbus/testnet1` it looks outdated (?)
- remove `lighthouse`s `load_testnet_config` in favor of `get_eth2_network_config` to centralize that logic (It had differences)
- some spelling

## Additional Info
Both the command of #1992 and the scripts of #1987 seem to work fine, same as `bn` and `vc`

common/eth2_network_config/build.rs

@@ -0,0 +1,63 @@
+//! Downloads a network configuration from Github.
+
+use eth2_config::{
+    altona, mainnet, medalla, pyrmont, spadina, toledo, Eth2NetArchiveAndDirectory,
+    GENESIS_FILE_NAME,
+};
+use std::fs::File;
+use std::io;
+use zip::ZipArchive;
+
+const ETH2_NET_DIRS: &[Eth2NetArchiveAndDirectory<'static>] = &[
+    altona::ETH2_NET_DIR,
+    medalla::ETH2_NET_DIR,
+    spadina::ETH2_NET_DIR,
+    mainnet::ETH2_NET_DIR,
+    pyrmont::ETH2_NET_DIR,
+    toledo::ETH2_NET_DIR,
+];
+
+fn main() {
+    for network in ETH2_NET_DIRS {
+        match uncompress_state(network) {
+            Ok(()) => (),
+            Err(e) => panic!(
+                "Failed to uncompress {} genesis state zip file: {}",
+                network.name, e
+            ),
+        }
+    }
+}
+
+/// Uncompress the network configs archive into a network configs folder.
+fn uncompress_state(network: &Eth2NetArchiveAndDirectory<'static>) -> Result<(), String> {
+    if network.genesis_is_known {
+        let archive_path = network.genesis_state_archive();
+        let archive_file = File::open(&archive_path)
+            .map_err(|e| format!("Failed to open archive file {:?}: {:?}", archive_path, e))?;
+
+        let mut archive =
+            ZipArchive::new(archive_file).map_err(|e| format!("Error with zip file: {}", e))?;
+
+        let mut file = archive.by_name(GENESIS_FILE_NAME).map_err(|e| {
+            format!(
+                "Error retrieving file {} inside zip: {}",
+                GENESIS_FILE_NAME, e
+            )
+        })?;
+        let path = network.dir().join(GENESIS_FILE_NAME);
+        let mut outfile = File::create(&path)
+            .map_err(|e| format!("Error while creating file {:?}: {}", path, e))?;
+        io::copy(&mut file, &mut outfile)
+            .map_err(|e| format!("Error writing file {:?}: {}", path, e))?;
+    } else {
+        // Create empty genesis.ssz if genesis is unknown
+        let genesis_file = network.dir().join(GENESIS_FILE_NAME);
+        if !genesis_file.exists() {
+            File::create(genesis_file)
+                .map_err(|e| format!("Failed to create {}: {}", GENESIS_FILE_NAME, e))?;
+        }
+    }
+
+    Ok(())
+}