Token definitions fetching improvements

[Nodonisko]

We should come up with better solution for fetching token definitions. We now have hardcoded eth, pol, bnb & sol which is suboptimal. Also it will fetch that definitions during startup which is also not good because parsing it is kind of slow.

• Only definitions for enabled networks and networks of imported accounts should be fetched. (DONE in feat(suite-native): only download token definitions for relevant networks #15263)
• [ ] Don't fetch token definitions right on app start, not to make it slow.

possible followups:
#15252 #15253

[matejkriz]

Notes for QA: check that token definitions are not missing when needed (tokens have fiat rates, are visible in transaction list)