Allow for PKCE browser flow for non SignIn TX types

[WietseWind]

Will work on this in due time :) Great idea!

@tequdev

[WietseWind]

Implementation:

Integrate alternative TX template for OAUTH platform, and then integrate in PKCE, and then update in Universal SDK
https://github.com/XRPL-Labs/XummPkce/blob/main/src/index.ts

[WietseWind]

We can actually re-use OAUTH2 platform,

https://oauth2.xumm.app/auth?response_type=token&client_id=xxx&state=yyy&scope=XummPkce&redirect_uri=zzz

... Already supports:
&force_network=XAHAU
signers=rwietsevLFg8XSmG3bEZzFein1g8RBqWDZ,rTeLeproT3BVgjWoYrDYpKbBLXPaVMkge

... We can add txhex / ... as extra param and use that in the OAuth sign flow