Add Lua embedding

[barrbrain]

Description

This adds a minimal Lua interface for interacting with a store. The motivating example transferring data by exercising FastSlowStore::get_part_unchunked().

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

local digests = {
  {"7465eeede3419aaa6def3cf3811bb4ce7416320da65fd60d6aa4d56cd3b4d0d3", 4},
  -- snip --
  {"2f1d593cd98cb5bf81eb9d880221122342784ac5fb7f41f2b137006bafc92e39", 3}
}
local count = #digests
for i = 1, count do digests[i] = digest(table.unpack(digests[i])) end
get_store("CAS_MAIN_STORE"):get_many(digests, 5, function (blob) print(blob and #blob) end)

Checklist

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

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This change is 

[CLAassistant]

All committers have signed the CLA.

[barrbrain]

Fixed the Bazel build by dropping the vendored feature from the mlua dependency and plumbing a bazel_dep. However, building with nix or cargo requires additional work.

[barrbrain]

Closing this draft in favour of a WebAssembly component embedding. The exploration of API shape was informative.

[MarcusSorealheis]

@barrbrain could you reference this PR in the WebAssembly component embedding?