Sign Solana transaction with selected account instead of fee payer

[yrliou]

We used to assume the wallet account will always be the fee payer and sign our Solana transactions with the fee payer account, which is not necessary the case, we should sign the transactions with the selected account instead.

One use case reported by https://civic.me/:

1. User requests our backend to issue a token
2. Our backend creates and signs a transaction (fee payer must be set at this point, and I think we set it as ourselves)
3. We send the serialised tx back to the user
4. User co-signs and sends to the chain.

In the above case, we should co-sign the transaction with the selected (connected) account.

[kjozwiak]

The above requires 1.46.139 or higher for 1.46.x verification 👍

[srirambv]

Verification passed on

Brave	1.46.141 Chromium: 108.0.5359.99 (Official Build) (64-bit)
Revision	410951fc34bb4b2cbf182231f9f779efaafaf682-refs/branch-heads/5359_71@{#9}
OS	Windows 11 Version 22H2 (Build 22623.1028)

• Verified steps from brave/brave-core#16154
• Verified able to submit a Sign and Send transaction
• Verified able to submit and confirm transaction on both software and hardware wallet

27051.mov

[srirambv]

Note: this issue can't be tested on Android since Dapp support is not yet enabled on Release. Its already checked on nightly so leaving the Android label as is