tiup client does not accept input on STDIN

[morgo]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

tiup client does not accept input on STDIN. This makes it hard to use as a replacement for mysql with any scripting.

2. What did you expect to see?

$ echo "show databases" | mysql
Database
INFORMATION_SCHEMA
METRICS_SCHEMA
PERFORMANCE_SCHEMA
mysql
test

3. What did you see instead?

$ echo "show databases" | tiup client
Starting component `client`: 
Connected with driver mysql (5.7.25-TiDB-v4.0.12)

4. What version of TiUP are you using (tiup --version)?

$ tiup --version
1.4.0 tiup
Go Version: go1.16.2
Git Ref: v1.4.0
GitHash: eb335e6ad9d0dc19d1c43db422cd409d62bc3cea