Improve startup speed

[marckhouzam]

What this PR does / why we need it

This PR avoids writing to the config files and the .data-store.yaml on startup if not necessary.

Writing to the config file to update the server list has a cost for each context. I ended up with 27 contexts configured, which triggered writing to file over 50 times which added 0.65 seconds on my M1 Mac, making the startup jump from around 0.35 seconds to a full second. Having the CLI take a full second to execute any command implies that each call to shell completion takes at least 1 second, which is slow for the user.

Updating the server list does not actually need to be done all the time.

Furthermore, the CLI currently writes to the .data-store.yaml and to the config files on every command when it stores the LastCLIVersionUsed. This file writes are also unnecessary most of the time.

This PR has two commits:

1. compute a sha of the contexts and another for the servers and store it in the data-store. Then for each command, instead of blindly synching contexts and servers, only do the sync if either sha has changed.
2. on write the last executed cli version to the data-store if the version is different than what is stored already, and only disable the features.global.context-target-v2 feature if it is currently enabled (to avoid writing to the config files)

This reduces the execution time for commands that don't access the backend; this means that shell completion for commands and flags becomes noticeably more responsive when a user has multiple contexts configured.

Before this PR, running tanzu version with 9 contexts configures takes around 0.5 seconds.
With this PR is becomes closer to 0.3 seconds.

Which issue(s) this PR fixes

Fixes #823

Describe testing done for PR

I use this little utility called entr which will execute a command whenever a specific file is changed.

# Monitor changes to the two config file and the datastore file
$ echo ~/.config/tanzu/config.yaml| entr echo config changed >>out.txt
$ echo ~/.config/tanzu/config-ng.yaml| entr echo config-ng changed >>out.txt
$ echo ~/.config/tanzu/.data-store.yaml | entr echo datastore changed >>out.txt
$ tail -f out.txt

Before the PR, if I run any command such as tanzu -h >/dev/null I see the config files change for every command multiple times (one for each context).

And for commands that trigger the lastTanzuVersion check (most commands), such as tanzu plugin list >/dev/null, I see the datastore file updated and the config files also.

With this PR, running those same commands, I don't see any changes done to the config or datastore files.

I also ran some normal shell completion down the command tree, and the responsiveness is noticeably better.

Release note

Improve CLI execution speed, especially for shell completion responsiveness

Additional information

Special notes for your reviewer

[vuil]

Overall optimization is quite clean and much appreciated, thanks for looking into this.

I have a very tiny concern if the skipping of the SetContext() has any unintended consequences, and if we want to cover our bases.

[anujc25]

LGTM. Very nice way to test the change as well. I just have one question.

[vuil]

Very nice approach of comparing the server and context contents!
lgtm!