Add support for custom avatars (via file path) and for not touching the already-set avatar

This is based on the work done in https://github.com/etkecc/baibot/pull/60 by https://github.com/Fmstrat (Ben Curtis),
with various changes on top to make the code more idiomatic and flexible.

This commit squashes the following patches (newest first):

- Improve handling of `user.avatar` configuration (null & empty string being the same now) and add support for a special `keep` value
- Minor import reordering
- Simplify avatar configuration (`user.avatar.source` -> `user.avatar`)
- Combine `logo_bytes` and `mime_type` determination logic and do not fall back to default avatar if reading the custom avatar file fails
- Switch from deprecated `mime_guess::guess_mime_type(avatar_path)` to `mime_guess::from_path(avatar_path).first_or_octet_stream()`
- Relax `mime_guess` constraint and order alphabetically
- Use `mime` from `mxlink`
- Add mime-type support and switch to user.avatar.source
- (Original work by Fmstrat) Add support for custom avatars

Co-authored-by: Fmstrat <nospam@nowsci.com>

src/bot/load_config.rs

@@ -5,7 +5,7 @@ use anyhow::anyhow;
 
 use crate::agent::AgentPurpose;
 
-pub use crate::entity::cfg::{Config, defaults as cfg_defaults, env as cfg_env};
+pub use crate::entity::cfg::{Avatar, Config, defaults as cfg_defaults, env as cfg_env};
 
 pub fn load() -> anyhow::Result<Config> {
     let config_file_path = env::var(cfg_env::BAIBOT_CONFIG_FILE_PATH)
@@ -37,6 +37,9 @@ pub fn load() -> anyhow::Result<Config> {
                 config.user.encryption.recovery_reset_allowed = value.parse::<bool>()?;
             }
             cfg_env::BAIBOT_USER_NAME => config.user.name = value,
+            cfg_env::BAIBOT_USER_AVATAR => {
+                config.user.avatar = Avatar::from_string(value);
+            }
             cfg_env::BAIBOT_COMMAND_PREFIX => config.command_prefix = value,
             cfg_env::BAIBOT_ROOM_POST_JOIN_SELF_INTRODUCTION_ENABLED => {
                 config.room.post_join_self_introduction_enabled = value.parse::<bool>()?;