A tool for debugging/tuning the configuration

We can provide a tool like pd-ctl for these purposes:

• Useful to tune the configuration of TiFlash
  Some configuration can be modified online, such as log level. But now we have to restart tiflash process after tiup cluster edit-config. Some users don't want to restart their process.
  Or we can support this feature in TiFlash to do this: Modify Configuration Online

• Useful to parse the DecodedTiKVKey
  Now if we happen to error like this:

(1105, '[FLASH:Coprocessor:BadRequest] Income key range [[7480000000000000385F72FFFFFFFFFFFFFFFF],[7480000000000000385F72FFFFFFFFFFFFFFFF00]) is illegal for region: 72')

To get the table_id and handle from these keys, we have to

# Go to cluster_manage/tikv_util, run a python with
>> import common
>> e = '7480000000000000385F72FFFFFFFFFFFFFFFF'
>> base64.b64encode(common.decode_pd_key(e, len(e)))
b'dIAAAAAAAAA4X3L//////////w=='
# Then parse it through tidb-ctl
>> tiup ctl tidb decoder 'dIAAAAAAAAA4X3L//////////wA='
format: table_row
table_id: 56
row_id: 9223372036854775807

Start by reviewing cluster_manage/tikv_util and the existing tiup ctl tidb decoder flow, then compare the proposed pd-ctl-like configuration tool with the online configuration approach linked in the issue. The scope needs to be decided between online TiFlash configuration, DecodedTiKVKey parsing, or both; done would mean a usable tool that supports the selected operations without requiring the current manual steps.