Support invoking some of the subcommands without tiup

Feature Request

Is your feature request related to a problem? Please describe:
For most enterprise users, they use tiup just to maintain a cluster. Most of the time, what they run are just the sub-commands of cluster components. Currently, cluster is a subcommand of tiup, it's recommended to run tiup cluster to maintain clusters according to official documents.

Always typing tiup cluster is inconvenient comparing to just typing cluster or ticluster apparently.

Describe the feature you'd like:
Support installing some of the subcommands as standalone commands.

We can learn from rustup. rustup is a similar tools as tiup, but it maintains components of rust. After using rustup to install toolchains, it is not required to run rustup cargo or rustup rustc to invoke cargo or rustc. It installs rustc and cargo as standalone commands and those commands will invoke configured version at runtime.

tiup can also install frequent used subcommands like cluster and playground as standalone commands, so user can choose what they want conveniently. And tiup becomes what it actually claims itself:

tiup is a tool to download and install TiDB components.

Describe alternatives you've considered:
Users can setup shell alias by themselves. It's unfriendly to users, but can solve the problem currently.

Teachability, Documentation, Adoption, Migration Strategy: