feat(wallet): add Base to preloaded EVM networks

[onyb]

Resolves brave/brave-browser#42885
Resolves brave/brave-browser#42884

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally:
  • npm run test -- brave_browser_tests, npm run test -- brave_unit_tests wiki
  • npm run presubmit wiki, npm run gn_check, npm run tslint
• 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/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

See STR in linked issue

[supermassive]

Let's avoid being it a netsted pref of brave.wallet.custom_networks since it's already a separate pref
Just brave.wallet.aurora_mainnet_migrated would be enough

Yes, there is brave.wallet.custom_networks.fantom_mainnet_migrated above but such approach causes problems when fromat of parent pref gets changed.

[supermassive]

const char*
std::string_view

[supermassive]

wallet core lgtm

[brave-builds]

Released in v1.75.131

[srirambv]

Verification passed on

Brave	1.75.142 Chromium: 132.0.6834.57 (Official Build) nightly (64-bit)
Revision	d6e67b9
OS	Linux

• Verified steps from issue description
• Verified Aurora is removed from default network list on a clean profile
• Verified if Aurora is set as default its retained as the the default network post upgrade
• Verified if Aurora is not set as default it is removed from the list of default network
• Verified Base is added as a default network on both clean and upgrade

27013.mp4