introduce only-decryption mode

[hehechen]

Signed-off-by: hehechen [email protected]

What problem does this PR solve?

Issue Number: ref pingcap/tiflash#5122

Problem Summary:

What is changed and how it works?

Check List

TiFlash Proxy introduce only-decryption mode. If TiFlash set --only-decryption, TiFlash Proxy will not startup TiKV server.
Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
  ./bin/tiflash/tiflash pagectl -V 3 -P /data2/chentongli/tidb-data/tiflash-19111/page/meta -I 0 --config_file_path conf/tiflash.toml
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None