add --create-manifest-if-needed flag

[JL03-Yue]

#15254

Add --create-manifest-if-needed flag
Create tool manifest automatically when installing a new local tool if the flag is on and no manifest exists.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[dsplaisted]

This looks good. The main thing still needed is to add test coverage.

[dsplaisted]

Per a discussion today, we may want to change the default so that you don't need to specify the --create-manifest-if-needed flag. For completeness we would probably add a flag to disable this behavior. If the --create-manifest-if-needed flag is not specified, then we probably do not want to create a manifest in the current folder; we would require there to be a .git or .sln file/folder.

[nagilson]

@baronfel Was a final decision reached on this change? I believe we are going to go with what this PR does for now?

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There is the proposed --manifest-file, but that was not added here to my knowledge.

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In addition, regarding:

--create-manifest-if-needed false

...

change the default so that you don't need to specify the --create-manifest-if-needed

I assume that with the below stipulation, this means it's by default IFF the following is true?

If the --create-manifest-if-needed flag is not specified, then we probably do not want to create a manifest in the current folder

[dsplaisted]

Looks great! A few suggestions still.

[baronfel]

@JL03-Yue we need to doc this! I just went looking and couldn't find it. I'll make a docs issue and we can work on doing that.

Logged dotnet/docs#39600 for this.