Tracking: basic multi-asset fee support

[hdevalence]

Is your feature request related to a problem? Please describe.

Users should be able to pay fees in non-native tokens (at a higher price), so that they can use Penumbra just by sending funds into the shielded pool.

Describe the solution you'd like

• fee: v0 support for multi-asset fees #4320 lays out basic multi-asset fee support at the protocol level, but leaves some threads dangling:
• Swap alt fee tokens into the native token prior to fee burn #4328
• Add events for fees
• Change Rust view server to record alt gas prices #4329
• Change Rust view server to report alt gas prices in RPC responses #4330
• Add alt fee token support to pcli #4331

Additional context

Description of the motivation and rationale can be found in #4320

[TalDerei]

Before the next sprint planning starts, should this be tagged?