refactor: make ReadableSize more readable.

[QuenKar]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

make ReadableSize debug format for better readable.
for example:
run cargo run -- standalone start, and we can see some log info.
previous format：

wal: WalConfig {
        dir: None,
        file_size: ReadableSize(
            268435456,
        ),
        purge_threshold: ReadableSize(
            4294967296,
        ),
        purge_interval: 600s,
        read_batch_size: 128,
        sync_write: false,
    },

current format：

wal: WalConfig {
        dir: None,
        file_size: 256.0MiB,
        purge_threshold: 4.0GiB,
        purge_interval: 600s,
        read_batch_size: 128,
        sync_write: false,
    },

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

Merging #2614 (2f550ef) into develop (3217b56) will decrease coverage by 0.37%.
The diff coverage is 100.00%.

❗ Current head 2f550ef differs from pull request most recent head 36b6b5b. Consider uploading reports for the commit 36b6b5b to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2614      +/-   ##
===========================================
- Coverage    85.14%   84.77%   -0.37%     
===========================================
  Files          738      738              
  Lines       118319   118322       +3     
===========================================
- Hits        100747   100312     -435     
- Misses       17572    18010     +438     

[killme2008]

LGTM 👍

[fengjiachun]

LGTM