Initial commit

[priteshbandi]

This is first code PR for this package and contains following changes

1. Copied notation-go/plugin/proto related code to notation-plugin-framework-go/plugin. Will delete notation-go code in subsequent PRs
2. Created new Cli package which provides boilerplate code required to generate plugin executable.
3. example package contains and example plugin implementation (some documentation is missing but will add in subsequent PRs

[rgnote]

LGTM

[Two-Hearts]

@priteshbandi could you help me understand better of the purpose of this repo again? (since this PR is just an initial one, it does not provide the whole picture of the repo.)
My understanding/questions:

1. The current notation-go library already provides all the necessary functions/APIs. Is there anything that's blocking our users to use notation-go directly at the moment? (I guess this answers why we should have this new repo.)
2. Which packages are you planning to move from notation-go? (I could see proto in this PR, anything else in your plan?)

/cc: @shizhMSFT @yizha1 @FeynmanZhou

[priteshbandi]

@Two-Hearts The library's objectives are as follows:

1. Simplifying the implementation process for plugin developers by encapsulating all overarching concerns.
2. Promoting a practice that enables plugin code to be utilized as a plugin library in notation-go, while also supporting the generation of executable.

The current notation-go library already provides all the necessary functions/APIs. Is there anything that's blocking our users to use notation-go directly at the moment? (I guess this answers why we should have this new repo.)

notation-go has lot of direct or indirect dependencies like oras-go, ldap, go-cose, jwt, etc. which plugin implementer don't need. When I think about dependency is unnecessary maintenance.

Which packages are you planning to move from notation-go? (I could see proto in this PR, anything else in your plan?)

I am planning to move Plugin Interface and its dependencies to this pkg. Basically all dependency required to use plugin as library.

[shizhMSFT]

LGTM for initial commits. We can improve others like license headers as well as license checkers in the next PRs.