refactor(x/tx)!: switch to interface in GetSignersContextOptions

[aaronc]

Description

Needed for #15284

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Author Checklist

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I have...

• included the correct type prefix in the PR title
• added ! to the type prefix if API or client breaking change
• targeted the correct branch (see PR Targeting)
• provided a link to the relevant issue or specification
• followed the guidelines for building modules
• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
• included comments for documenting Go code
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Reviewers Checklist

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I have...

• confirmed the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
• confirmed all author checklist items have been addressed
• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage
• manually tested (if applicable)

[facundomedica]

utACK

[julienrbrt]

utACK

[kocubinski]

Tested ACK! Same changes works in #15380 (with tests)

[aaronc]

Tagged x/tx/v0.3.0 with this