Add dry-run option to update-dependencies tool and pipelines

[lbussell]

Fixes #6389

[mthalman]

The descriptions of these options already states that it will skip the PR creation if they are not provided:

dotnet-docker/eng/update-dependencies/CreatePullRequestOptions.cs

Lines 29 to 31 in 438dab2

	new Option<string>("--user") { Description = "GitHub or AzDO user used to make PR (if not specified, a PR will not be created)" },
	new Option<string>("--email") { Description = "GitHub or AzDO email used to make PR (if not specified, a PR will not be created)" },
	new Option<string>("--password") { Description = "GitHub or AzDO password used to make PR (if not specified, a PR will not be created)" },

So it seems we already have a means to skip the PR creation and don't need a new option. Are those descriptions not true? If not, we should consider providing that functionality anyway since it would make sense when executing the tool from a dev machine where you wouldn't be providing the creds.

[lbussell]

The descriptions of these options already states that it will skip the PR creation if they are not provided:

dotnet-docker/eng/update-dependencies/CreatePullRequestOptions.cs

Lines 29 to 31 in 438dab2

	new Option<string>("--user") { Description = "GitHub or AzDO user used to make PR (if not specified, a PR will not be created)" },
	new Option<string>("--email") { Description = "GitHub or AzDO email used to make PR (if not specified, a PR will not be created)" },
	new Option<string>("--password") { Description = "GitHub or AzDO password used to make PR (if not specified, a PR will not be created)" },

So it seems we already have a means to skip the PR creation and don't need a new option. Are those descriptions not true? If not, we should consider providing that functionality anyway since it would make sense when executing the tool from a dev machine where you wouldn't be providing the creds.

See #6414 (comment). Although, after thinking about it some more, the driver behind this change is not to test the code between the credentials short-circuit and the PR creation. So in order to keep things simpler I think yours and Justin's idea makes more sense.