opm index -- refactor global flags support

[grokspawn]

in dialog #986 (comment) we learned that opm index sets help/usage support for 'global flags' as persistent cobra flags.

Since opm index is to be deprecated because it only handles a deprecated SQLite index format, we need to refactor the flags support.

Instead of using persistent flags set at the opm root, we need to have each subcommand locally implement support for the use-http and skip-tls-verify.

Preliminary file list (under cmd hierarchy):

• ./opm/internal/util/util.go
• ./opm/render/cmd.go
• ./opm/registry/add.go
• ./opm/index/cmd.go
• ./opm/alpha/veneer/basic.go
• ./opm/alpha/bundle/unpack.go