Attempt to address some of the concerns raised in #86

• add the HTTP verb to the log to differentiate which endpoint is actually used
• use structured logging, so it will be easier to process the log afterward. For instance, structured logging allows data to be a key value inside a json
• move the span from the top-level function to the inner wrapper, so we don't get to see the parameters. Parameters can contain sensitive information (prevent leaking unseal key #85) and tends to bloat the logs. Users can still log the parameter if they want by adding a trace themself.
• remove some info traces that were redundant or did not contains useful information

Here there is an example to the new log, which can be useful to compare with the one in #85 . Here, RUST_LOG="info,rustify=off" is used, as it won't likely to be fixed upstream.

Unhappy path:

2024-03-27T08:06:02.413716Z  INFO request{method=GET path=/sys/health}: vaultrs::api: start request
2024-03-27T08:06:02.422478Z ERROR request{method=GET path=/sys/health}: vaultrs::api: error=An error occurred with the request

Happy path:

2024-03-27T08:06:00.283379Z  INFO request{method=POST path=/sys/init}: vaultrs::api: start request