Add units to the numeric options in the configuration file

Feature Request

Is your feature request related to a problem? Please describe:

Most of the numeric configs lack unit. It is easy to confuse users. For example,

https://github.com/pingcap/tidb/blob/b05327509cb3584881815c2605a7537bbc24a8f0/config/config.toml.example#L243

104857600 should be 104857600B and 10737418240 should be 10737418240B or 10G.

Here is another example causing problems: #17434. https://github.com/pingcap/tidb/issues/17434#issuecomment-640209382

Describe the feature you'd like:

• Add units to the numeric options in the configuration file.
• Forbid numeric options without units.

Describe alternatives you've considered:

• Explain the units in documents
• Explain the units in the configuration file comments

Both of these solutions are not easy to use enough.

Teachability, Documentation, Adoption, Migration Strategy:

Change how the configuration file is parsed.