Add extension support

[BarDweller]

This PR contains a first pass at adding support for Extensions (https://github.com/buildpacks/spec/blob/main/image_extension.md) to libcnb.

This introduces

• extension.go with Extension/ExtensionInfo structs that represent extension.toml content
• generate.go/generate_test.go to represent the generate lifecycle invocation
• example_generate_test.go as an example of GenerateFunc
• Renames Main to BuildpackMain and introduces ExtensionMain (breaking change)

• Adds extension env var constants to platform.go

  This PR does NOT address

• the current min/max supported buildpack API level.. (this does likely need updating to encompass extensions)
  • (these are currently defined in build.go but should probably move to a common location for build/generate)
• any sort of utility function for implementors of extensions to assist in writing Dockerfile content

• any commonality between BuildpackInfo and ExtensionInfo

  This PR will go a long way towards resolving Issue #230

  Raising this at this stage mostly for comments, happy to address additional items from above in this PR if the consensus is that makes sense =)

[dmikusa]

@samj1912 - Definitely want to get your thoughts on this, if you have time.

[BarDweller]

Updated to address the test/lint issues from previous, and rebased to main.

[dmikusa]

Looks good. Thanks @BarDweller I was able to use this to make a basic extension.

I do want to take a pass at a couple of the things we discussed on Slack but that'll be a separate PR.

@samj1912 - Let me know if you have any thoughts or I'll go ahead and merge this in on Monday