Update Gemini widget Oauth flow to use PKCE

[ryanml]

Gemini now supports PKCE, so we we should use a code challenge and verifier as part of the authentication process.

STR/Cases:

Run through both Gemini/Binance regression checks as per brave/brave-core#6306 (comment). Example via brave/brave-core#6306 (comment).

[srirambv]

Verification passed on

Brave	1.13.79 Chromium: 85.0.4183.69 (Official Build) dev (64-bit)
Revision	4554ea1a1171bd8d06951a4b7d9336afe6c59967-refs/branch-heads/4183@{#1426}
OS	Linux

• Verified test plan from issue description
• Verified Gemini flow as per Refresh token should be used in re-authorization flow (Gemini) #10978 (comment)
• Verified Binance widget works as expected
• Verified able to connect to the widget without any issues
• Verified Summary, Deposit, Convert and Buy tabs are displayed after connecting the widget and summary is in hide mode
• Verified disconnecting/reconnecting via the widget worked as expected and displayed the correct balance
• Verified that deposit addresses are being displayed correctly under the Deposit tab
• Verified QR code images under the Deposit tab correctly enlarges/displays the QR codes
• Verified tokens such as XRP display both a deposit address and memo address
• Verified selecting Learn More under the menu opens https://brave.com/binance
• Verified selecting tokens via the Buy tab correctly updates the Buy [Token] button
• Verified clicking on the Buy button under the Buy tab correctly opens a new tab with the correct amounts already filled out

---

Verification passed on

Brave	1.13.79 Chromium: 85.0.4183.69 (Official Build) dev (64-bit)
Revision	4554ea1a1171bd8d06951a4b7d9336afe6c59967-refs/branch-heads/4183@{#1426}
OS	Windows 10 OS Version 1809 (Build 17763.1397)

• Verified test plan from issue description
• Verified Gemini flow as per Refresh token should be used in re-authorization flow (Gemini) #10978 (comment)
• Verified Binance widget works as expected
• Verified able to connect to the widget without any issues
• Verified Summary, Deposit, Convert and Buy tabs are displayed after connecting the widget and summary is in hide mode
• Verified disconnecting/reconnecting via the widget worked as expected and displayed the correct balance
• Verified that deposit addresses are being displayed correctly under the Deposit tab
• Verified QR code images under the Deposit tab correctly enlarges/displays the QR codes
• Verified tokens such as XRP display both a deposit address and memo address
• Verified selecting Learn More under the menu opens https://brave.com/binance
• Verified selecting tokens via the Buy tab correctly updates the Buy [Token] button
• Verified clicking on the Buy button under the Buy tab correctly opens a new tab with the correct amounts already filled out

---

Verification passed on

Brave	1.13.79 Chromium: 85.0.4183.69 (Official Build) dev (64-bit)
Revision	4554ea1a1171bd8d06951a4b7d9336afe6c59967-refs/branch-heads/4183@{#1426}
OS	macOS Version 10.15.5 (Build 19F101)

• Verified test plan from issue description
• Verified Gemini flow as per Refresh token should be used in re-authorization flow (Gemini) #10978 (comment)
• Verified Binance widget works as expected
• Verified able to connect to the widget without any issues
• Verified Summary, Deposit, Convert and Buy tabs are displayed after connecting the widget and summary is in hide mode
• Verified disconnecting/reconnecting via the widget worked as expected and displayed the correct balance
• Verified that deposit addresses are being displayed correctly under the Deposit tab
• Verified QR code images under the Deposit tab correctly enlarges/displays the QR codes
• Verified tokens such as XRP display both a deposit address and memo address
• Verified selecting Learn More under the menu opens https://brave.com/binance
• Verified selecting tokens via the Buy tab correctly updates the Buy [Token] button
• Verified clicking on the Buy button under the Buy tab correctly opens a new tab with the correct amounts already filled out