Change configuration format to TOML

This introduces several improvements to the agent configuration.

- The file format is modified from INI style files to TOML formatted
  files.
- The configuration is constructed using an hierarchical organization
  following the order:
  - The default hardcoded values are loaded
  - If a configuration file is found in /usr/etc/keylime-agent.conf, it
    is loaded and any option set overrides the default values
  - Load configuration snippets from /usr/etc/keylime-agent.conf.d/*,
    overriding the previous values
  - Load the configuration from /etc/keylime-agent.conf, overriding
    previous values
  - Load configuration snippets from /etc/keylime-agent.conf.d/*,
    overriding previous values
  - Finally any configuration option can be overriden via environment
    variable, by using the 'KEYLIME_AGENT_' prefix. For example,
    KEYLIME_AGENT_PORT=9003 will override the 'port' option from the
    'agent' section with the value '9003'.

This was introduced by using an added dependency, the 'config' crate
using the 'toml' feature for the toml file format.

Signed-off-by: Anderson Toshiyuki Sasaki <[email protected]>

Author: ansasaki

Cargo.lock

@@ -23,7 +23,9 @@ base64 = "0.13"
 cfg-if = "1"
 clap = { version = "~3.1.18", features = ["derive"] }
 compress-tools = "0.12"
+config = { version = "0.13", features = ["toml"] }
 futures = "0.3.6"
+glob = "0.3"
 hex = "0.4"
 libc = "0.2.43"
 log = "0.4"
@@ -39,6 +41,7 @@ serde_json = { version = "1.0", features = ["raw_value"] }
 static_assertions = "1"
 tempfile = "3.0.4"
 tokio = {version = "1.13.1", features = ["full"]}
+toml = "0.5"
 tss-esapi = "7.1.0"
 thiserror = "1.0"
 uuid = {version = "0.8", features = ["v4"]}