Add basic Federated Learning functionality to browser

[LorenzoMinto]

Re-opened from https://github.com/brave/brave-core/pull/12668 Resolves https://github.com/brave/brave-browser/issues/21536 Privacy Review: https://github.com/brave/reviews/issues/1249

This PR adds basic federated learning (FL) client functionality to the brave_federated component. When some eligibility criteria are met (connected to power and on unmetered network), the client requests a list of available tasks from the server. Tasks are either Train or Evaluate: Train will start the training of a simple linear regression model on local data, while Evaluate will simply do a forward step and compute the loss/accuracy of the existing model. Both Train and Evaluate tasks provide model parameters that are used locally by the client. To support training in the client, some basic linear algebra primitives were implemented. Once the results has been locally computed, it’s sent back to the server. Task results structure is defined in transport.proto: in the case of training the trained model (vector of floats) is shared back with the server, while for evaluation what gets shared is a loss/accuracy metric (float).

NB: This feature sits behind the BraveFederated feature flag.

Submitter Checklist:

• [x] I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• [x] There is a ticket for my issue
• [x] Used Github auto-closing keywords in the PR description above
• [x] Wrote a good PR/commit description
• [x] Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• [x] Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• [x] Checked the PR locally:
  • npm run test -- brave_browser_tests, npm run test -- brave_unit_tests wiki
  • npm run lint, npm run presubmit wiki, npm run gn_check, npm run tslint
• [x] Ran git rebase master (if needed)

Reviewer Checklist:

• [ ] A security review is not needed, or a link to one is included in the PR description
• [ ] New files have MPL-2.0 license header
• [ ] Adequate test coverage exists to prevent regressions
• [ ] Major classes, functions and non-trivial code blocks are well-commented
• [ ] Changes in component dependencies are properly reflected in gn
• [ ] Code follows the style guide
• [ ] Test plan is specified in PR before merging

After-merge Checklist:

• [ ] The associated issue milestone is set to the smallest version that the changes has landed on
• [ ] All relevant documentation has been updated, for instance:
  • [ ] https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • [ ] https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • [ ] https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • [ ] https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • [ ] https://github.com/brave/brave-browser/wiki/Custom-Headers
  • [ ] https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • [ ] https://github.com/brave/brave-browser/wiki/QA-Guide
  • [ ] https://github.com/brave/brave-browser/wiki/P3A

Test Plan: