CLI: Cherry-pick changes from main to support TMC plugins

[giri-varma]

What this PR does / why we need it

Changes included in this PR:

1. Add support for TMC-Plugin discovery Adding support for TMC-Plugin discovery  #2950
2. Add context command CLI: Add context command #2202
3. Acquire lock before reading tanzu config file to avoid possible misconfigured file reads Acquire lock before reading tanzu config file to avoid possible misconfigured file reads #3177
4. Add target separation CLI: Add target separation #3126

Which issue(s) this PR fixes

Fixes #3204, #2200, #3175, #2201

Describe testing done for PR

Manually tested the same way as the original PRs. This change shall be picked up for backwards compatibility testing in TKG.

Release note

* Added discovery source for TMC in Tanzu CLI. Also, fixed fetching artifacts over http to handle large file reads by processing chunks at a time.

* Added a context command as part of the Tanzu CLI to create and manage contexts in the CLI.

* Acquire lock before reading tanzu config file to avoid races with config writes.

* Added support for two targets (context types) in Tanzu CLI: `kubernetes`, `mission-control`. This is currently backwards compatible, i.e., the plugins are still available at the root level. In addition to that, we have all the contextual plugins grouped under their corresponding targets.

Note: This is currently behind a feature flag. To enable the flag please run tanzu config set features.global.context-target true and then run `tanzu init` once after the upgrade.

PR Checklist

Please ensure the following:

• Use good commit messages
• Ensure PR contains terms all contributors can understand and links all contributors can access
• Squash the commits into one or a small number of logical commits

Additional information

Special notes for your reviewer

This is not a ground-up change. Am only cherry-picking the below commits from main

1. Add support for TMC-Plugin discovery git cherry-pick 1775a2720cdfe2791169468e0526f0cee39f7ed9
2. Add context command git cherry-pick 7a25ddcc63116e2661aebd7a8f8d2eb10aff3e3f
3. Acquire lock before reading tanzu config file to avoid possible misconfigured file reads 0c14d3ff167e0ce8f792ea28f1b8bb0774316fad
4. Add target separation git cherry-pick 48a7073624975f972392c9fdb730d290043dd181

[github-actions]

Cluster Generation A/B Results:
https://storage.googleapis.com/tkg-clustergen/3373/20220915230315/clustergen.diff.txt
Author/reviewers:
Please review to verify that the effects on the generated cluster configurations are exactly what the PR intended, and give a thumbs-up if so.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-0.25@7131595). Click here to learn what that means.
The diff coverage is n/a.

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[giri-varma]

Closed in favor of #3385