Add PIT HLD document , this PR replace previous PR#1014.

[clarklee-guizhao]

Replace previous PR#1014, #1014.

• Add clarification for platform plugins, add plan for supporting platform APIs.
• Fix some typo.
• Change branch name to master.

Repo	PR title	State
sonic-buildimage	[sonic-pit] Add PIT(Platform Integration Test) feature, first commit with only framework and 2 test cases with 1 supportting platform.
sonic-buildimage	[sonic-pit]Add PIT(Platform Integration Test) feature, second part, add 6 test cases.

[clarklee-guizhao]

Replace previous PR#1014, #1014.

• Add clarification for platform plugins, add plan for supporting platform APIs.
• Fix some typo.
• Change branch name to master.

Repo PR title State
sonic-buildimage [sonic-pit] Add PIT(Platform Integration Test) feature, first commit with only framework and 2 test cases with 1 supportting platform.
sonic-buildimage [sonic-pit]Add PIT(Platform Integration Test) feature, second part, add 6 test cases.

@zhangyanzhao @lihuay @keboliu
Hi folks, I redo this HLD PR to replace previous #1014 PR, please help review this PR for PIT, thanks a lot.

[clarklee-guizhao]

Also, please add @jeff-yin and @prgeor as reviews as PR#1014 :)

[zhangyanzhao]

@StormLiangMS this feature need be back ported to 202211

[zhangyanzhao]

@clarklee-guizhao can you please sign the EasyCLA? That is required before the PR can be merged. Also can you please address Jeff's comments? Thanks.

[zhangyanzhao]

@jeff-yin @lihuay can you please check this PR to see if you are ok to approve and merge? Thanks.

[jeff-yin]

Just one comment about something minor to double check. Otherwise LGTM.

[zhangyanzhao]

added to 202305 release plan