feat(x/auth/tx): Allow for Custom SignModeHandlers.

[itsdevbear]

Description

Closes: #XXXX

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[julienrbrt]

utACK, depinject changes lgtm.

[itsdevbear]

@tac0turtle this code works, we've been using it in our repo for a while now. Though I think at some point a broader txconfig refactor is needed.

[tac0turtle]

could you update it to main and we will enable auto merging. The changes look good to me

[itsdevbear]

yup can do in a bit, just on my phone

[itsdevbear]

@tac0turtle merged up