feat: users namespace

[gfanton]

Following and closing #384

This PR adds a user check when adding a new package. The check is based on the first part of the realm path: gno.land/(r|p)/<user_namespace>. It calls gno.land/r/demo/users.GetUserByName(<namespace>) to ensure the user exists and has registered the user namespace.

This PR also adds restricted name ability.

• Only the admin can add restricted names
• Restricted names CAN be registered using an admin invite

thanks @anarcher for the base

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests
• Added new benchmarks to generated graphs, if any. More info here.

[codecov]

Codecov Report

Attention: Patch coverage is 27.90698% with 62 lines in your changes missing coverage. Please review.

Project coverage is 54.92%. Comparing base (a7fd05b) to head (5df88c5).

Files	Patch %	Lines
gno.land/pkg/sdk/vm/keeper.go	17.85%	44 Missing and 2 partials ⚠️
gno.land/pkg/integration/testing_integration.go	63.63%	4 Missing and 4 partials ⚠️
gno.land/pkg/sdk/vm/errors.go	0.00%	6 Missing ⚠️
gno.land/pkg/integration/testing_node.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2471      +/-   ##
==========================================
- Coverage   54.95%   54.92%   -0.04%     
==========================================
  Files         595      595              
  Lines       79568    79646      +78     
==========================================
+ Hits        43727    43743      +16     
- Misses      32533    32588      +55     
- Partials     3308     3315       +7     

Flag	Coverage Δ
contribs/gnofaucet	15.31% <ø> (ø)
contribs/gnokeykc	0.00% <ø> (ø)
contribs/gnomd	0.00% <ø> (ø)
gno.land	62.54% <27.90%> (-0.60%)	⬇️

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[ajnavarro]

Mostly good.

For the record, I'm still thinking that this logic should live outside the VMKeeper.

[piux2]

For better security and trust of the chain, we should consider removing the admin from the system contract entirely. No one should be able to add a contract under /r/system unless it is voted on by the gov DAO or through a chain upgrade that involves consent from the validators.