[rush] Add installation for subspaces

[william2958]

Summary

This MR adds the ability to install dependencies for a subspace.

Details

This MR adds the following changes:

• Modifies where rush looks for installation and configuration files for an installation to be configurable for a specific subspace. For example, rush can now look for configuration files for a specific subspace under common/config/subspaces/first_subspace as opposed to the default common/config
• Adds the support for the splitWorkspaceCompatibility option in the subspace configuration, which allows subspaces to specify their configuration files under the project folder itself if it is the only project in a subspace. For example, if project a is the only project in subspace first_subspace, it can have it's configuration files under the folder a/subspace/first_subspace instead of the default common/config/subspaces/first_subspace folder.
• Allows configuration for where rush creates the /temp/ folder and all corresponding files that go under the temp folder. For example, rush can now install for a subspace under the /temp/first_subspace folder as opposed to the default /temp folder
• Add the selector --subspace <subspace_name> for the commands rush install and rush update to install for a specific subspace.
• Add support for filtered installs (using selectors like --to, --from, etc.) by installing all affected subspaces sequentially.
• Add the selector for subspace tags used for building a specific workspace, e.g. rush build -o subspace:first_subspace
• Disable checking for the link flag if subspaces are enabled (to be fix later)
• Insert custom pnpmfile logic that links projects that are not within the same subspace.
• Support common-versions logic within subspaces
• Support repo-state checking within subspaces

How it was tested

• Tested the enabled feature using https://github.com/william2958/rush-subspace-prototype
• Tested the disabled feature using https://github.com/microsoft/rush-example
• @octogonz reviewed the code to ensure all new functionality is properly gated when the feature is disabled

Impacted documentation

Rush operation should be completely unaffected if the subspaces feature not enabled.

However since the feature is close to code complete, we're going to start work on website docs soon.

[william2958]

Note that the ignore-scripts PR features have been removed from this PR to separate the concerns. There appears to be more design decisions and reviews required the the other PR, so this PR is just to update the way rush looks for configuration files and where it installs dependencies to support subspaces.