feat: Implement SIP-29 permission and handlers

[FrederikBolding]

Add an endowment:assets permission that lets Snaps expose OnAssetsLookup and OnAssetsConversion. These handlers are going to be called by controllers responsible for managing non-EVM assets. This PR is mostly adding support for those handlers, as well as the types/structs required to validate the parameters and responses. Additionally some work was done to transform the responses to filter out any out of scope assets returned.

For reference: https://metamask.github.io/SIPs/SIPS/sip-29

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.80%. Comparing base (277b974) to head (cb67728).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3028      +/-   ##
==========================================
+ Coverage   94.76%   94.80%   +0.04%     
==========================================
  Files         500      503       +3     
  Lines       10911    10996      +85     
  Branches     1672     1685      +13     
==========================================
+ Hits        10340    10425      +85     
  Misses        571      571              

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