Don't block Solana AddUnapprovedTransaction with fee estimation RPCs

[yrliou]

Description

After brave/brave-core#23214, we blocked Solana AddUnapprovedTransaction with fee estimation RPCs.
When RPCs are very slow, users won't see anything because we only popup the panel after AddUnapprovedTransaction is done.
We should consider change this to first add the unapproved transaction without waiting, and fire an transaction changed event to frontend once fee estimation is ready.

Steps to reproduce

1. create a Solana send transaction with slow network

Actual result

transaction approval screen won't show up after quite a while

Expected result

transaction approval screen should show up in a timely manner

Reproduces how often

Easily reproduced

Desktop Brave version (brave://version info)

nightly

Android device

All

Channel information

• release (stable)
• beta
• nightly

Reproducibility

• with Brave Shields disabled
• with Brave Rewards disabled
• in the latest version of Chrome

Miscellaneous information

No response