Integrate `flexbuild` for flex as a sub-command of `gsctl`

[yecol]

as titled. move the functionalities in flexbuild in /flex to gsctl flexbuild

[lidongze0629]

Usage:

$ gsctl flexbuild --help

Options:
--graphscope-repo -g (optional)
    GraphScope code repository location.

--app, -a (required)
    Application type
    Allowed: db, olap, ldbcdriver, gnn

--output-dir -o (optional)
   Output prefix

Arguments:
  COMPONENTS...
    Components (LEGO bricks) to include and link
    Allowed: hiactor, gaia, pregel, pie, flash, grape-cpu, grape-gpu, tensorflow, pytorch, vineyard, mcsr, gart, graphar