Add optional file logging, and add logging for heartbeat disconnects

[Trogluddite]

This change adds:

1. Optional logging to a file
2. a log event when 'Heartbeat Timeout' type disconnects occur

Testing

Existing unit tests pass; no new tests added.

Basic functional testing (listed below) was made with release builds on these targets:

aarch64-apple-darwin
x86_64-unknown-linux-gnu

• server can start
• client can connect to server
• interrupted connection (severed VPN between client & server) causes heartbeat failure and disconnect
• server drops socket on graceful client disconnect
• default 'trace logs to stdout' behavior is preserved with escape sequences coloring output
• new 'trace logs to file' behavior functions as expected (sans escape sequences).

Test build using (on both linux and MacOS):

rustc 1.85.1 (4eb161250 2025-03-15)
LLVM version: 19.1.7

New Dependencies:

time = "0.3.35"
tracing-appender = "0.2.3"

[Trogluddite]

Example log events:

2025-03-26T21:40:35.281814Z  INFO control{addr=172.31.11.42:49554}: bore_cli::server: connection dropped after heartbeat timeout peer_addr="172.31.11.42:49554"
2025-03-26T21:40:35.281852Z  INFO control{addr=172.31.11.42:49554}: bore_cli::server: connection exited

This just gives us a little more info about why the client disconnected.