Enable increasing tracing level without increasing the log level

Currently, to enable debug level tracing, one sets the environment variable RUST_LOG to debug. This aloso increases the log level to be debug which often is not desired. The request is to add a new, appropriately named environment variable, such as RUST_TRACE, which can be used to independently change the tracing level.

From scanning the code I believe this is why, (1) is where tracing is configured. It uses this function (2) which uses RUST_LOG, if it could just read from RUST_TRACE instead that would give us what we want. However, (2) is also used at (3) for logging, so it could be changed to have the log level passed in and read from different env var for log vs trace:

1. https://github.com/quickwit-oss/quickwit/blob/03ed8cfb9adc4c24cb0af3bad53c78442f11e1de/quickwit/quickwit-cli/src/logger.rs#L334
2. https://github.com/quickwit-oss/quickwit/blob/03ed8cfb9adc4c24cb0af3bad53c78442f11e1de/quickwit/quickwit-cli/src/logger.rs#L46
3. https://github.com/quickwit-oss/quickwit/blob/03ed8cfb9adc4c24cb0af3bad53c78442f11e1de/quickwit/quickwit-cli/src/logger.rs#L73

Start in quickwit/quickwit-cli/src/logger.rs at the tracing setup around line 334 and the shared configuration function around lines 46 and 73. Trace how RUST_LOG currently controls both tracing and logging, then define the separate environment-variable behavior requested. Done means tracing can be increased without also increasing the log level.