Implement get_tree() feature

[boldpulse]

Description

I handled get_tree() GRPC request in the CAS Server. Used FIFO(First-In-First-Out) traversal method ( BFS Algorithm ) to loop over directories using VecDeque data structure. Paging is supported and page_token in GetTreeRequest is used to point the directory to start traversing with. And next_page_token in GetTreeResponse refers the page_token parameter which will be used in the next request.

Fixes #355

Type of change

Please delete options that aren't relevant.

• [x] New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Still not tested yet.

Checklist

• [x] Updated documentation if needed
• [ ] Tests added/amended
• [x] bazel test //... passes locally
• [x] PR is contained in a single commit, using git amend see some docs

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This change is 

[boldpulse]

+@allada, @MarcusSorealheis

[adam-singer]

@aleksdmladenovic can we add tests ?

[MarcusSorealheis]

This looks good. thank you for the PR aleksdmladenovic!

[MarcusSorealheis]

@allada If this looks good still, please merge it.Even though we don't use it, a lot of tools do use it so maybe it will be helpful to NativeLink users.

[MarcusSorealheis]

@aleksdmladenovic what was the reason for the most recent push?

[MarcusSorealheis]

Looks like it may have been a rebase but correct me if I am wrong.

[boldpulse]

Just rebase. cc: @MarcusSorealheis