Add a subcommand to create a project environment variable

[threepipes]

Checklist

=========

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have checked for similar issues and haven't found anything relevant.
  • Support Project Secrets / Environment Variables #652 is the issue that this PR is targeting
• This is not a security issue (which should be reported here: https://circleci.com/security/)
• I have read Contribution Guidelines.

Internal Checklist

• I am requesting a review from my own team as well as the owning team
• I have a plan in place for the monitoring of the changes that I am making (this can include new monitors, logs to be aware of, etc...)

Changes

=======

• Add one of the project subcommands
  • circleci project secret create <vcs> <org> <project> <env-name>

Rationale

=========

This PR partially solves #652.
And this PR is the subsequent change to #770.

Examples

============

$ go run main.go project secret create gh OrgName PrjName foo
Enter the environment variable value and press enter ●●●●●●●
Environment variable name=foo value=xxxxest already exists. Do you overwrite it? Yes
+----------------------+---------+
| ENVIRONMENT VARIABLE |  VALUE  |
+----------------------+---------+
| foo                  | xxxxenv |
+----------------------+---------+

$ go run main.go project secret create gh OrgName PrjName bar --env-value somevalue
+----------------------+----------+
| ENVIRONMENT VARIABLE |  VALUE   |
+----------------------+----------+
| bar                  | xxxxalue |
+----------------------+----------+

$ go run main.go project secret create gh OrgName PrjName bar --env-value somevalue
Environment variable name=bar value=xxxxalue already exists. Do you overwrite it? No
Canceled

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[threepipes]

I referred https://pkg.go.dev/google.golang.org/grpc#CallOption to make this option.

[threepipes]

I added this interface for mock input on tests.